ASTM D2270

Viscosity index

Viscosity index is a single number for how hard an oil’s kinematic viscosity falls between 40°C and 100°C. A higher VI falls less. ASTM D2270, which agrees with ISO 2909, is the calculation. It is not a quality grade, and it says nothing about additives, oxidation, or extreme-pressure performance.

What goes in

U is the kinematic viscosity at 40°C. Y is the kinematic viscosity at 100°C. Both come from ASTM D445, ASTM D7042, ISO 3104, or IP 71. From Y, the method takes two reference oils that have that same 100°C viscosity: L is the 40°C viscosity of the VI 0 reference, and H is the 40°C viscosity of the VI 100 reference.

When U is greater than H, the oil is at VI 100 or below:

VI = (L − U) / (L − H) × 100

When U is less than H, the oil is above VI 100. N = (log H − log U) / log Y, and

VI = (10^N − 1) / 0.00715 + 100

When U equals H, the VI is 100. The result is rounded to the nearest whole number. A value ending in exactly 0.5 rounds to the even number, so 116.5 is reported as 116. ASTM D2270 does not define a VI below 2.0 mm²/s at 100°C.

Published examples

ASTM D2270 works an oil at 73.30 mm²/s and 40°C and 8.86 mm²/s at 100°C. The table gives L = 119.94 and H = 69.48, and the VI is 92. An oil at 22.83 and 5.05 mm²/s, with H = 28.975, is VI 156. An oil at 53.47 and 7.80 mm²/s, with H = 57.31, is VI 111.

Up to 70 mm²/s at 100°C, the standard’s Table 1 is the referee. The calculator on this site uses the Appendix X2 quadratics ASTM published for computers, which the standard puts within 0.1% on L and H. Above 70 mm²/s it uses the Section 5 equations. If a quadratic and the table ever disagree, the table wins.

What a maintenance tech should do with it

• Mineral industrial gear oils often land around VI 90 to 100. PAO gear oils often land around 140 to 160. Those are typical bands, not limits.
• A higher VI holds more of its 40°C viscosity when the sump is hot, so the operating viscosity is higher for the same ISO VG. That can be the reason to change chemistry instead of climbing a grade and paying for it in churning at startup.
• A multigrade engine oil’s VI is partly the viscosity-index improver. Shear down in a gearbox and the working VI is lower than the fresh-oil calculation. Gear oils specified under SAE J306 have their own shear test for this reason.
• VI from temperatures other than 40°C and 100°C is an estimate. Project those points with ASTM D341 first. ASTM D2270 calls that result information, not a value for a specification.

The converter does both steps and labels which one you got. The effect on a named grade is drawn on the VG 220, VG 320, and VG 460 pages.

Questions

Is a higher viscosity index always a better oil?

It is a flatter viscosity-temperature curve between 40°C and 100°C. The gearbox still needs the right viscosity at the temperature it runs, and it still needs the additives and the seal compatibility the OEM called out.

Why do two labs sometimes differ by a point or two?

The calculation itself is exact. The VI moves when the two kinematic measurements move. ASTM D2270’s reproducibility for petroleum base stocks in the usual range is 2 VI points.