On my car, my engine, my modifications, all of my oils show about a 10-20% loss in 100C viscosity at the end of an OCI vs. the VOAs for the same oils that I've done. Likely some of that is fuel b/c Blackstone doesn't test for it specifically and some of that is that I run ethanol blends. In fact, I got more with M1 than any other oil I've used/my data set. Who knows that that means and I'm not saying there is anything wrong with any oils I've used, but I've never had any oil stay in the 40W grade over a change in my Sportwagen. I drive it hard and it's modified so more than stock boost etc.