Lubricants: oil viscosity & grease grade
An oil's grade is its viscosity at 40 °C; what matters at the machine is its viscosity where it works. A VG 68 slideway oil (68 and 8.6 cSt at 40 and 100 °C) is 28.5 cSt at 60 °C.
Enter the two viscosities from the oil's data sheet and the working temperature; the oil is drawn on a viscosity-temperature chart, where its line is straight. Switch to grease for the NLGI grade from a worked penetration.
SAE grades by the 100 °C viscosity alone: a comparison, not an approval.
- ISO VG
- Viscosity at 50 °C
- SAE engine grade
- 20
- SAE gear grade
- 80
How it’s calculated
ASTM D341 (Walther): log log(ν + 0.7) = A − B log T, T in kelvin, ν in cSt; A and B through the 40 °C and 100 °C viscosities. The ISO grade is the 40 °C midpoint, ±10 %; the SAE grades go by the 100 °C viscosity.
ISO viscosity grades, at 40 °C
| ISO VG | Min, cSt | Mid, cSt | Max, cSt |
|---|---|---|---|
| 2 | 1.98 | 2.2 | 2.42 |
| 3 | 2.88 | 3.2 | 3.52 |
| 5 | 4.14 | 4.6 | 5.06 |
| 7 | 6.12 | 6.8 | 7.48 |
| 10 | 9.00 | 10 | 11.0 |
| 15 | 13.5 | 15 | 16.5 |
| 22 | 19.8 | 22 | 24.2 |
| 32 | 28.8 | 32 | 35.2 |
| 46 | 41.4 | 46 | 50.6 |
| 68 | 61.2 | 68 | 74.8 |
| 100 | 90.0 | 100 | 110 |
| 150 | 135 | 150 | 165 |
| 220 | 198 | 220 | 242 |
| 320 | 288 | 320 | 352 |
| 460 | 414 | 460 | 506 |
| 680 | 612 | 680 | 748 |
| 1000 | 900 | 1000 | 1100 |
| 1500 | 1350 | 1500 | 1650 |
SAE single grades, at 100 °C (engine 2021, gear 2019)
| SAE | Oil | Min, cSt | Under, cSt |
|---|---|---|---|
| 8 | engine | 4.0 | 6.1 |
| 12 | engine | 5.0 | 7.1 |
| 16 | engine | 6.1 | 8.2 |
| 20 | engine | 6.9 | 9.3 |
| 30 | engine | 9.3 | 12.5 |
| 40 | engine | 12.5 | 16.3 |
| 50 | engine | 16.3 | 21.9 |
| 60 | engine | 21.9 | 26.1 |
| 65 | gear | 3.8 | 5.0 |
| 70 | gear | 5.0 | 6.5 |
| 75 | gear | 6.5 | 8.5 |
| 80 | gear | 8.5 | 11.0 |
| 85 | gear | 11.0 | 13.5 |
| 90 | gear | 13.5 | 18.5 |
| 110 | gear | 18.5 | 24.0 |
| 140 | gear | 24.0 | 32.5 |
| 190 | gear | 32.5 | 41.0 |
| 250 | gear | 41.0 | – |
What does ISO VG mean?
The ISO viscosity grade is the oil's kinematic viscosity at 40 °C in cSt (mm²/s), rounded to a series where each grade is about half as thick again as the one below. An oil within ±10 % of the grade's midpoint carries it, so VG 68 covers 61.2 to 74.8 cSt.
Are SAE and ISO grades the same?
No. SAE engine and gear grades go by the viscosity at 100 °C (and, for W grades, by cold tests), ISO grades by 40 °C. Two oils of one ISO grade can fall in different SAE grades if their viscosity changes differently with temperature. The comparison here is by viscosity only.
What is an NLGI grade?
The NLGI number is a grease's consistency: a standard cone is dropped into the grease at 25 °C after it has been worked, and the depth it sinks, in tenths of a millimetre, places it in a grade from 000 (fluid) to 6 (very hard). NLGI 2 is the usual bearing grease.