The 0w-, 5w-, 10w-, etc, part of the
SAE viscosity grades defines the maximum allowable dynamic(absolute) viscosity at particular temperatures for each grade.
So roughly speaking, it defines the lowest temperature you should attempt to use that grade for.
The -20, -30, -40, etc, is defines the minimum/maximum, low shear-rate allowed kinematic viscosity at 100C(212F), and high shear rate kinematic viscosity at 150C. So roughly speaking, it defines the viscosity the oil should have at operating temperatures.
To determine the viscosity at some other temperature, you need it's viscosity measurements at two temperatures like 40C and 100C to get the oils viscosity index. Using that information you can get a good estimate of the viscosity at a lower temperature as long as it's not to close to the lower limit.
Temperature/Viscosity Conversion Calculator(not ASTM)
For example, let's run some numbers for M1 and Mobil Drive Clean. (Viscosities in cSt)
code:
M1 0w30 M1 5w30 MDC 5w30(dino)
100C 10.3 10 10.5
40C 56 56 62
VI 175 167 159
0C(32F) 419.1 445.6 535
-10C(14F) 846 923 1144
So, in this case, down to 14F, the difference is probably negligible among the synthetics, but starts to be significant between the synthetics and the dino, but not enough to be concerned.
Hope that helps.
[ December 07, 2004, 05:09 PM: Message edited by: 427Z06 ]
