Join Date: Dec 2011
Vehicle: Dodge Rear End Parts
Modifications: Nitro Gears, Auburn Posi's, HD Diff Covers, Solid Spacers
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Most oils today are 'Multigrade' oils, which means that the oil is defined by 2 viscosity grades (i.e. 10w-40, 5w-30, etc). Multigrades were developed more than 50 years ago to avoid the requirement of using thick oil in summer and thin oil in winter. The W stands for winter, not weight, which means that the oil has a certain viscosity/flow at low temperature. A lower 'W' number means the oil performs better in cold temperatures. The non 'W' number indicates the oil viscosity level at 100°C (212°F).This is a fixed limit, all oils that end in the same number (30) must meet the limits.
We recommend using 85w-140 in differentials. Whether you are changing out a differential cover, or just replacing the fluid, it's important to use the right oil in your diffs.