What each search box means

Company

Who makes the oil. Pick one or more, or leave it blank to see every maker.

The refiner whose name is on the pail: Castrol, Chevron, Mobil or Shell. If your machine manual names a brand, start here. If it names only a grade, leave this blank. The same ISO grade from another maker is usually an acceptable substitute, and the Attributes and AGMA Class boxes tell you whether it carries the same protection.

Application

The job the oil is sold for: gear, hydraulic, compressor, spindle, way, turbine. A gear oil in a hydraulic pump foams; a hydraulic oil in a loaded gearbox scuffs.

Oils are blended for a job, and the additive package is what differs. Pick the job your machine does.

• Gear: enclosed gearboxes. Usually carries extreme-pressure additives so the teeth do not weld under shock.
• Hydraulic: pumps, cylinders and valves. Carries anti-wear additives and is kept clean and foam-free.
• Bearing: circulating oil for plain and rolling bearings, often with little or no additive.
• Compressor: resists oxidation at the high temperatures inside an air compressor and drains water readily.
• Spindle: very thin oil for high-speed machine-tool spindles.
• Ways: tacky oil that clings to machine-tool slideways and stops stick-slip.
• Turbine: rust-and-oxidation inhibited, long life, separates water fast.
• Cutting Fluid, Paper Machine, Pneumatic, Gas Engine, Rust Preventative: specialist products, named for the job.

ISO Grade

How thick the oil is at 40 °C. Bigger number, thicker oil: 32 is a thin hydraulic oil, 220 a typical gearbox fill, 680 very heavy.

The ISO viscosity grade (ISO VG) is the single number most manuals give. It is the oil's thickness at 40 °C in centistokes, and each grade is about half as thick again as the one before: 2, 3, 5, 7, 10, 15, 22, 32, 46, 68, 100, 150, 220, 320, 460, 680, 1000, 1500. A grade covers plus or minus ten percent, so a 46 is any oil between 41.4 and 50.6 cSt.

Match the grade your manual calls for. If you have no manual, the rule of thumb is faster and cooler means thinner, slower and hotter and heavier-loaded means thicker. Choosing your oil viscosity walks through it, and the conversion chart turns an SAE or AGMA number into an ISO grade.

Attributes

The additive package. Anti-Wear for pumps, Extreme Pressure for loaded gears, R&O for turbines and bearings, Food-Grade, Biodegradable, Fire-Resistant.

• Anti-Wear (AW): zinc or ashless additives that protect pump parts under moderate load. The standard for hydraulics.
• Extreme Pressure (EP): sulphur-phosphorus additives that stop steel welding to steel where gear teeth meet under shock. The standard for enclosed gearboxes. EP additives can attack brass and bronze, so check Copper Compatibility if the gearbox has a bronze worm wheel.
• R&O: rust and oxidation inhibited, nothing else. Turbines, circulating systems and lightly loaded gears.
• Food-Grade: registered for incidental food contact (NSF H1).
• Biodegradable: ester or vegetable base for sites where a leak reaches soil or water.
• Fire-Resistant: water-glycol or phosphate-ester fluids for hydraulics near furnaces and hot metal.

Pour Point

The coldest temperature the oil still flows at. Your coldest start-up should be at least 5 °C warmer than this.

Cool the oil until it stops moving; the last temperature at which it still poured is the pour point. A pump cannot draw oil that will not flow, so the AGMA gear-lubrication standard wants the pour point at least 5 °C below the coldest temperature the machine will ever be started at. Mineral oils sit around minus 15 to minus 30; synthetics reach minus 40 to minus 60. The box lists only values the maker has published, and the count under the heading says how many of the 613 products that is.

Flash Point

The temperature at which the oil's vapour will light from a flame. Higher is safer near hot surfaces. A safety number, not a performance number.

Heat the oil in an open cup and pass a flame over it; the temperature at which the vapour first flashes is the flash point. It tells you how hot the oil can get before it becomes a fire risk, and nothing about how well it lubricates. Industrial oils run 200 to 260 °C. The AGMA gear standard sets a floor of 180 °C for the thinner grades and 200 °C above ISO 68. A used-oil sample with a flash point far below the datasheet has fuel or solvent in it.

AGMA Class

The AGMA 9005-F16 lubricant class the maker claims: RO rust-and-oxidation inhibited, AS antiscuff (EP), CP compounded (worm gears). Blank means not claimed.

The American Gear Manufacturers Association sorts enclosed-gear lubricants into three classes, and a gearbox manual will often name one.

• RO: rust and oxidation inhibited. Lightly loaded, no shock.
• AS: antiscuff, carries extreme-pressure additives. Most industrial gearboxes.
• CP: compounded with fatty material for the sliding contact in worm gears.

The box shows a class only where the maker states it on the datasheet, which is why the count is low. To check a product against the tables yourself, use Evaluate against AGMA 9005-F16 under Thresholds. Choosing your oil viscosity explains the standard.

Base Oil (API Group)

The API base-stock group. I, II and III are mineral; IV is PAO; V is everything else (PAG, ester). Few makers publish it.

The American Petroleum Institute sorts base stocks into five groups by how they were made. I is solvent-refined mineral, II and III are hydrocracked mineral (cleaner, higher VI), IV is polyalphaolefin (PAO), and V is a catch-all for everything else, which includes both polyglycol (PAG) and ester. Because V lumps together oils that do not mix, this box is for matching a specification; for choosing an oil use Base Oil Type.

Base Oil Type

The three families that decide a fill. Mineral: the default. PAO: a synthetic that mixes with mineral. PAG: a synthetic that does NOT mix with the other two, and is the best choice for worm gears.

Three chemistries cover nearly every industrial oil, and the choice between them is the one that costs money if it is wrong.

• Mineral: refined from crude. The default, the cheapest, and what most gearboxes were filled with at the factory. Viscosity index about 95 to 105.
• PAO (polyalphaolefin): a synthetic hydrocarbon. Chemically the same family as mineral oil, so it mixes with it, needs no flush, and is kind to seals and paint. Buys a higher VI (130 to 150), a lower pour point and a longer drain. The safe synthetic upgrade.
• PAG (polyalkylene glycol): a synthetic built on ether oxygens, which makes it slippery in a way hydrocarbons are not. Worm gears run cooler and 10 to 30 percent more efficiently on PAG, and it holds a very high VI. The catch: PAG does not mix with mineral or PAO, so a change needs a full drain and flush, and some paints and seals object to it. Never top up one with the other.

The box lists only products whose maker states the base stock, so the count is low; a product with the Synthetic box set but no base type is a synthetic of unstated chemistry, and the datasheet link on its row is where to look.

Micropitting (FVA 54)

The FVA 54 micropitting test. "High" means the oil protects case-hardened gear flanks from grey staining. Matters for wind, mill and heavily loaded helical gears.

Micropitting is a grey, frosted wear on hardened gear teeth that slowly destroys the profile. The German FVA 54 test runs a gear pair through rising load stages and grades the oil's resistance as low, medium or high. For most gearboxes it does not matter. For large, slow, heavily loaded helical and planetary gears, wind turbines and mill drives among them, the manual will ask for a high rating, and this box finds the oils that publish one.

Rust Test (D665)

ASTM D665 rust test with water in the oil. A is distilled water; B is synthetic seawater and harder to pass. Pass means a steel pin stayed clean.

A steel pin spins in the oil with water added for 24 hours. Procedure A uses distilled water, procedure B synthetic seawater, which is the harsher test. A product that lists B has been shown to protect steel with salt water in the system; a product that lists A, with fresh water. The box shows which procedure the maker published; every product in it passed.

Viscosity Index

How little the oil thins as it heats. Higher holds its thickness over a wider temperature swing. Mineral about 95 to 105; synthetic 130 to 200.

Every oil gets thinner when hot. The viscosity index (VI) is a score for how much: a higher number means less change between 40 °C and 100 °C. A machine that starts cold and runs hot wants a high VI so the oil is pumpable at start and still thick enough at temperature. Plain mineral oils score about 95 to 105, PAO synthetics 130 to 150, PAG synthetics 150 to 250. All about viscosity index has the detail, and the VI calculator works it out from two readings.

Copper Compatibility

ASTM D130 copper-strip rating. 1A or 1B is clean. 2 and above means the additives can attack brass and bronze: worm wheels, bushings, coolers.

A polished copper strip sits in the hot oil for three hours and is then compared with a colour chart. 1A and 1B are barely tarnished; 2 is moderate; 3 and 4 are dark. Active extreme-pressure additives are what cause the tarnish, and the same chemistry will eat a bronze worm wheel or a brass oil cooler over time. If the machine has yellow-metal parts in the oil path, keep to 1A or 1B. Lubricant-induced copper corrosion explains why.

Synthetic

Conventional (refined from crude) or Synthetic (built from chemicals). Use the Base Oil Type box when you need to know which synthetic.

Conventional means the base oil was refined from crude. Synthetic means it was manufactured, which buys a higher viscosity index, a lower pour point and a longer drain interval, at a higher price. This box is the maker's own one-word description. When the choice matters, especially for a worm gear, use Base Oil Type instead, because not all synthetics mix with each other.

Viscosity at 40 °C

The measured thickness at 40 °C, in cSt. This is the number the ISO grade is built on.

Kinematic viscosity at 40 °C in centistokes, the figure the maker publishes on the datasheet. The ISO grade is this number rounded to the nearest standard grade. Use it when a manual gives a viscosity rather than a grade, or when you want to be sure a 220 is a thin 220 or a thick one.

Viscosity at 100 °C

The measured thickness at 100 °C, in cSt. Shows how much the oil thins when hot.

Kinematic viscosity at 100 °C in centistokes. Together with the 40 °C figure it gives the viscosity index. A gearbox that runs at 80 or 90 °C is closer to this number than to the ISO grade, which is why two oils with the same grade can behave differently hot.

FZG fail stage, min

The FZG scuffing test: the load stage, 1 to 14, that the gear teeth survived before scuffing. 12 or more is the usual gearbox requirement. Enter the lowest you will accept.

The FZG rig runs a pair of test gears through load stages of rising torque until the teeth scuff. The last stage passed is the result; the common form is "A/8.3/90", meaning the A gears at 8.3 metres per second and 90 °C. Stage 12 is the usual minimum for an industrial extreme-pressure gear oil, and many synthetics reach 14, the top of the scale. Some makers publish "greater than 12", which the table stores as a minimum. Enter the lowest stage you will accept and products below it, or with no published result, drop out.

Viscosity 40 °C

A range in cSt. Narrower than an ISO grade: use it when a manual gives a viscosity rather than a grade.

Enter a minimum, a maximum, or both, in centistokes at 40 °C. An ISO grade already covers plus or minus ten percent, so this is for the case where the manual gives a viscosity and you want the oils nearest to it.

Viscosity 100 °C

A range in cSt at 100 °C, for a machine that runs hot.

Enter a minimum, a maximum, or both, in centistokes at 100 °C. Use it when the operating temperature is nearer 100 than 40 and the hot viscosity is what the bearing or gear actually sees.

Viscosity Index, min

The lowest VI you will accept. 95 keeps plain mineral oils in; 130 keeps only synthetics and premium minerals.

Enter the lowest viscosity index you will accept. The AGMA gear standard asks for 90 as a floor. A machine with a wide swing between cold start and running temperature benefits from 130 or more.

Pour Point °C, max

Enter your coldest start-up temperature minus 5 °C. Oils that would not flow at that temperature drop out.

Enter the highest pour point you will accept, as a temperature. The rule is coldest start-up minus 5 °C: for a machine started at minus 10 outdoors, enter minus 15. Products with a higher pour point, or none published, drop out.

Flash Point °C, min

The lowest flash point you will accept. 180 is the AGMA floor for thin grades, 200 for ISO 68 and up.

Enter the lowest flash point you will accept. Use the AGMA floor, 180 °C for grades up to ISO 68 and 200 °C above, unless the machine runs near hot metal, in which case ask for more.

Evaluate against AGMA 9005-F16

Pick the AGMA class your gearbox needs and every result gets a verdict column: does its published data meet Table 2 (RO), 3 (AS) or 4 (CP).

Choose RO, AS or CP and the result table grows a column that checks each product's published numbers against the matching table in AGMA 9005-F16: viscosity index, pour point, flash point, rust, copper, foam and FZG stage. "Meets" means every published figure clears the bar; "fails" names the figure that does not; "incomplete" means the maker did not publish something the table needs. A verdict is only as good as the datasheet behind it, so an incomplete is a reason to open the datasheet link, not a reason to reject the oil.

Only products that publish an FZG stage

Hide products whose maker never published an FZG scuffing result.

Most makers publish an FZG result only for their gear oils, and some not at all. Tick this to see only products with a number, which is the honest list when scuffing matters. The count beside the FZG box says how many of the 613 that is.

Available on Amazon

Show only oils you can buy today, in a 1 or 5 gallon pack, with a buy button on the row.

Tick this and the results are limited to products with a verified Amazon listing in a 1 gallon or 5 gallon pack. Every link was opened and the listing read before it was stored, so the button leads to the product and grade on the row, not a neighbour. Products without a button can still be bought: the datasheet link leads to the maker, and most are sold through distributors.

EHL Film

How much oil film each candidate would put between the teeth, as a ratio: 1.00 is the thinnest in the list. No units. From the oil alone; the gearbox is held constant.

Gear teeth do not touch when things are going well: a film of oil a fraction of a micrometre thick is squeezed between them, and it is the oil's viscosity, and the way that viscosity rises under pressure, that build it. AGMA 925 gives the equation (Dowson and Toyoda). For one gearbox at one load every geometry term in it is the same for every oil, so what is left is the oil: viscosity at your operating temperature to the 0.69 power, times the pressure-viscosity coefficient to the 0.56 power.

The column shows that quantity as a ratio to the thinnest film in the list, so 1.59 means 59 percent more film than the weakest candidate. It has no units. The pressure-viscosity coefficient comes from the base oil type (mineral, PAO or PAG, 925 Table 4), so an oil whose base stock is not published gets no number. Hover a value for the viscosity and coefficient behind it. Two things it is not: an absolute thickness, and a lambda ratio. Describe the gear pair in step 7 and you get both.

hc, film thickness

The oil film between the teeth at the pitch point, in micrometres (thousandths of a millimetre). AGMA 925 central film, isothermal.

Once the gear pair is described (teeth, module, face width, speed, load), the AGMA 925 equation can be solved for real: the result is the central film thickness h_c at the pitch point, in micrometres. Industrial gears run on films of 0.1 to 2 µm. The page gives the isothermal value at the pitch point; the standard works 25 points along the tooth and corrects for the heat generated as the oil enters the contact, and both lower the number a little. Its own worked example: 0.24 µm at the pitch point, 0.23 at the minimum, 0.14 with the heating correction.

Lambda, specific film thickness

Film thickness divided by the combined roughness of the two tooth surfaces. No units. Below about 1 the high spots touch; above about 3 the surfaces are fully apart.

A film is only thick enough in relation to how rough the surfaces are. Lambda is the film divided by the combined roughness of pinion and gear (the root of the sum of the squares of the two Rq values, adjusted to the width of the contact). Below about 1 the asperities carry part of the load and you are in boundary lubrication, relying on the additives. Between 1 and 3 is mixed. Above 3 the surfaces are fully separated and wear is governed by fatigue, not contact. Roughness comes from the finish you choose in step 7, using the as-manufactured mid-range of AGMA 925 Table 3; ground teeth at Rq 0.8 µm are the default.

Wear risk

Probability of wear-related distress at this lambda and speed, from AGMA 925 Figure 19. Green at 5 %, amber to 40 %, red above. Floored at 5 %.

How much lambda you need depends on speed: slow gears tolerate a thinner film than fast ones. AGMA 925 Figure 19 draws the lambda at which 5, 40 and 80 percent of gears showed wear-related distress, against pitch line velocity, and clause 12 turns that into a probability for any lambda. The page prints it as a percentage: green at 5 percent (the floor the standard's own procedure applies), amber up to 40, red above. It is a probability of distress over the gear's life, not a prediction of failure tomorrow, and it assumes the film and roughness are as computed.

Tooth and contact temperature

The flank runs hotter than the sump, and hotter still for an instant at each tooth contact. Tooth temperature is sump plus 0.47 of the peak flash; contact temperature adds the flash at each point.

Oil in the sump is one temperature; the tooth flank is hotter because the mesh makes heat faster than the flank sheds it, and at the instant two teeth rub there is a further spike, the flash temperature, that lasts microseconds. AGMA 925 clause 8 computes the flash at every point of the contact from the sliding speed, the load and the steel's thermal properties (Blok's equation); it is zero at the pitch point, where there is no sliding, and highest near the start of single-tooth contact. The tooth temperature is the sump temperature plus 0.47 of that peak (Eq. 74), and it is the temperature the oil viscosity is taken at, so a hotter mesh thins the film twice over. The contact temperature at any point is tooth plus flash, and its maximum is what scuffing is judged against.

Scuffing risk

Probability that the oil film breaks down and the teeth weld locally: the peak contact temperature against the oil's scuffing temperature, AGMA 925 clause 10. Under 10 % low, 10 to 30 moderate, over 30 high.

Scuffing is sudden: the film collapses, asperities weld and tear, and the flank is damaged in one pass. It is governed by temperature, not by film thickness, which is why 925 treats it separately from wear. Each oil has a scuffing temperature: for oils that publish an FZG A/8.3/90 fail stage it comes from that stage (Eq. 108, with a base-oil factor), otherwise from the viscosity grade (Eq. 105 for plain R&O oils, Eq. 106 for antiscuff oils with extreme-pressure additives, which buys about 55 °C). The page compares the maximum contact temperature with it through a Gaussian whose spread is 15 percent of the mean and prints the probability, floored at 5 percent. Table 9: under 10 percent low, 10 to 30 moderate, over 30 high. 925 says the mineral-oil formulas are a conservative estimate for synthetics; the FZG route is the better one, and the stage may be reduced by one to allow for an oil ageing between changes.