Originally Posted by Draco-REX

The basics: Viscosity is the measure of how thick a liquid is. Oils have a higher viscosity than water (they flow slower). A higher viscosity oil helps keep moving parts apart, but too high and your oil system may have trouble pumping it and the oil will have trouble flowing into the smaller crevices that it needs to lubricate evenly.

Regular un-enhanced oils are rated by their viscoity, the 'weight'. This is the W number. 10W- 5W- etc. (For the record, W stands for Winter, not weight. The number is the viscosity of the oil at 0C [32F]. ) Multi-viscosity Oils have two numbers (i.e. 10W30).

Now, as oils heat up, their viscosity goes down. A motor oil that will flow properly for your engine when cold will be useless at operating temp. So they add long molecules called polymers that are all bunched up when cold, but unwind when hot. That increases the viscosity.

So, the second number in 10W30 is the equivalent viscosity at 100C [212F]. So a 10W30 oil at 100C is the same viscosity as a 30W oil at 100C.

The balance you need is an oil that's thin enough to flow properly when the engine is cold, but thick enough at operating temps to maintain the needed viscosity.

EDIT: Forgot to mention. There's a lot ofargument about the really diverse multi-viscosity oils (i.e. 0W50). A lot of people think the longer polymer chains needed to get a spread like that can be broken easier, so you need to change it more often in high-load applications.

Originally Posted by amstel78

With my M3, I changed tranny and diff oil at 1200 miles per manufacturer recommendation. I figured changing the drive train fluids on the Evo wouldn't hurt either... and in this case, I'm glad I did. The tranny, diff, and t-case oil was very dark, and there was a lot of sludge and magnetic residue around the drain bolts.