Originally posted by SCHU1:
I always thought the bottom number on the oil was what mattered.
The bottom number (e.g. 5W) is the "winter" weight of the oil. This is the viscosity of the oil at cold temperatures (or upon startup in colder weather).
The top number is the "summer" weight of the oil. It is the viscosity of the oil when the engine gets to operating temperatures.
Thus, both numbers are important, but one number might become
more important depending on where you live and what season you are currently in (i.e., winter or summer).
