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.

For differentials we recommend a 85w-140.