Originally Posted By: JayBB
Please explain this, how the viscosity could change and why it's even something to talk about..
Viscosity typically gradually goes down a little (like kv100 drops -2 cSt) the first 5,000 miles or so, and then after that starts to go back up.
Reduction in visc (kv100) is due to the VII (visc improver) molecules actually breaking apart over time and stress and/or some fuel dilution entering the sump from leaky fuel injectors and/or worn rings. An increase in visc is from oxidation (via heat stress and time), and is a sure sign the oil needs to be changed if kv100 gets to say, +2 cst over the new oil's value. ............. You used the high HTHS BMW 5w30, which means 12 cSt kv100 was what it was new, right in the SAE 30 weight range OK, indicating some VII breakdown and oxidation was equally acting to reduce and increase for a net gain of zero.
Note: HTHS is a higher temperature, high bearing speed (shear) viscosity, higher stress than a kv100, which affects oil film thickness the most, something to look up sometime when you're bored and want to read geeky stuff.