SAE J306
SAE gear oil grades vs cSt
SAE J306 classifies axle and manual-transmission lubricants. The high-temperature half of a grade is a kinematic viscosity window at 100°C. The winter half, the grades with a W, is a cold Brookfield limit. One centistoke reading can place the oil in the 100°C window. It cannot award the W.
The limits below follow SAE J306 as revised February 2019. The minimum at 100°C also has to be met after the 20-hour CEC L-45-A-99 shear test. Earlier editions used different 75W and 80W minima (4.1 and 7.0 mm²/s).
| SAE | 150,000 cP max, °C | Min cSt | Max cSt |
|---|---|---|---|
| 70W | -55 | 3.8 | — |
| 75W | -40 | 3.8 | — |
| 80W | -26 | 8.5 | — |
| 85W | -12 | 11 | — |
| 65 | — | 3.8 | < 5 |
| 70 | — | 5 | < 6.5 |
| 75 | — | 6.5 | < 8.5 |
| 80 | — | 8.5 | < 11 |
| 85 | — | 11 | < 13.5 |
| 90 | — | 13.5 | < 18.5 |
| 110 | — | 18.5 | < 24 |
| 140 | — | 24 | < 32.5 |
| 190 | — | 32.5 | < 41 |
| 250 | — | 41 | none |
The converter’s SAE gear line uses only the non-winter rows, and only when you tell it the viscosity is at 100°C. A 40°C ISO VG number is a different measurement.
Engine oil is a different standard
SAE J300 is the engine-oil classification. There is no SAE 90 engine oil. An SAE 90 gear oil (13.5 to less than 18.5 mm²/s at 100°C) overlaps SAE 40 and SAE 50 engine oils on kinematic viscosity and still is not an engine oil. The additives, the shear test, and the cold test are different, and the gearbox or the engine was approved on one of them.
| SAE | Min cSt | Max cSt | HTHS min, cP |
|---|---|---|---|
| 8 | 4 | < 6.1 | 1.7 |
| 12 | 5 | < 7.1 | 2 |
| 16 | 6.1 | < 8.2 | 2.3 |
| 20 | 6.9 | < 9.3 | 2.6 |
| 30 | 9.3 | < 12.5 | 2.9 |
| 40 | 12.5 | < 16.3 | 3.5 or 3.7 |
| 50 | 16.3 | < 21.9 | 3.7 |
| 60 | 21.9 | < 26.1 | 3.7 |
SAE 8, 12, 16, and 20 overlap on kinematic viscosity on purpose. High-temperature high-shear viscosity at 150°C (ASTM D4683) separates them. SAE 40 has two HTHS floors: 3.5 cP with 0W-40, 5W-40, and 10W-40, and 3.7 cP with 15W-40, 20W-40, 25W-40, and a straight SAE 40.
| SAE | Cranking, ASTM D5293 | Pumping, ASTM D4684 | Min cSt at 100°C |
|---|---|---|---|
| 0W | 6200 cP at -35°C | 60,000 cP at -40°C | 3.8 |
| 5W | 6600 cP at -30°C | 60,000 cP at -35°C | 3.8 |
| 10W | 7000 cP at -25°C | 60,000 cP at -30°C | 4.1 |
| 15W | 7000 cP at -20°C | 60,000 cP at -25°C | 5.6 |
| 20W | 9500 cP at -15°C | 60,000 cP at -20°C | 5.6 |
| 25W | 13000 cP at -10°C | 60,000 cP at -15°C | 9.3 |
Multigrades
A 75W-90 meets the 75W cold limit and the SAE 90 kinematic window. An 80W-90 does the same with the 80W cold limit. The 100°C viscosity of either oil is somewhere inside 13.5 to less than 18.5 mm²/s. Two oils with the same SAE grade can sit at opposite ends of that window, which is wide enough to matter in a gearbox that was developed on one of them.
Industrial enclosed gears are usually specified as an ISO VG, not an SAE grade. The path from pitch-line velocity to that ISO VG is the gearbox page. Branded gear-oil comparisons are on oilviscositychart.com.
Questions
What is SAE 90 in centistokes?
SAE J306 (2019) puts SAE 90 at 13.5 mm²/s minimum and less than 18.5 mm²/s, kinematic viscosity at 100°C.
Is SAE 90 thicker than SAE 140?
No. SAE 140 is the heavier gear grade: 24.0 to less than 32.5 mm²/s at 100°C. SAE 90 is 13.5 to less than 18.5.
Can I convert an ISO VG to an SAE grade directly?
Only after you know the viscosity at 100°C. ISO VG is a 40°C window. SAE J306 is a 100°C window. The viscosity index connects them, and two oils in one ISO VG can fall in different SAE grades.