Short answer
SAE J306 is the SAE standard that defines viscosity grades for automotive gear lubricants used in axles and manual transmissions. W grades such as 75W are defined by low-temperature performance and the other grades such as 90 or 140 by viscosity at 100°C. A multigrade like 75W-90 meets both. J306 grades describe thickness only, not performance level such as API GL-5.
How the grades are defined
Winter grades are set by the highest temperature at which the oil reaches a viscosity of 150,000 centipoise in a Brookfield test (ASTM D2983); that is the point where gear oil becomes too thick to circulate well. The limit is −55°C for 70W, −40°C for 75W and −12°C for 85W, with 80W in between.
The non-W grades are set by kinematic viscosity at 100°C (ASTM D445). In the current table, SAE 90 runs from 13.5 to under 18.5 cSt, SAE 110 from 18.5 to under 24.0 cSt and SAE 140 from 24.0 to under 32.5 cSt. The scale runs on through 190 and 250. J306 also sets a shear-stability requirement so an oil stays in its grade after a standardized shear test.
Reading a multigrade gear oil
A 75W-90 oil must meet the 75W low-temperature limit and also fall within the SAE 90 range at 100°C. That combination lets one oil flow at cold start and still hold the film thickness the gear designer expected at operating temperature. A 75W-140 or 80W-140 oil is thicker hot, which is why heavy towing axles and some performance axles list a 140 grade.
Single grades such as SAE 90 or SAE 250 are still used in some industrial, heavy-duty and older applications where cold flow is not a concern.
Gear grades are not engine oil grades
Gear oil numbers and engine oil numbers come from different standards and are not comparable by the digits. An SAE 90 gear oil (13.5 to under 18.5 cSt at 100°C) overlaps much of the SAE 40 engine oil range under SAE J300 (12.5 to under 16.3 cSt). A gear oil labeled 75W-90 is not “thicker than” a 15W-40 engine oil just because 90 is larger than 40.
What the 2019 revision changed
The February 2019 revision of J306 added three lower-viscosity grades, SAE 65, 70 and 75, to cover the thinner fluids some newer axles and transmissions use, and it narrowed the SAE 80 range. If a newer vehicle lists one of those grades, it is not an error; match it exactly rather than defaulting to a familiar 75W-90.
Frequently asked questions
Is 75W-90 thicker than 75W-85?
At 100°C, yes: SAE 90 has a higher viscosity range than SAE 85. Both share the 75W low-temperature limit.
Can I use 75W-140 instead of 75W-90?
Only if the vehicle maker allows it. Some axles list both depending on towing or climate; others specify one grade.
Does the viscosity grade tell me if the oil is GL-5?
No. SAE J306 covers viscosity only. The API designation, such as GL-5, is a separate performance claim.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
