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Some of the old formulations did, especially if the car had emissions issues. The interesting thing, this thickening never seems to effect wear rates.
Viscosity change indicates physical property of the oil has changed. UOA, as good as it is, captures partial information on oil, for example, it does not reveal how much mpg lost due to oil thickening.
True enough - but the very slight thickening from a HEAVY 30 (as the oils used to be) to a LOW 40, probably didn't change MPG all that much. People used to love to use and abuse this characteristic. It bothered me a lot too. But in 99.9% of the cases it really wasn't a bother to the engine in extended OCI's. In 10K or less OCI's the issue was a non-starter.
But Amsoil listened, and changed the formula. I think to this day is it was a well know phenomena and Amsoil's suppliers (Mobil in this case) probably weren't of much help. (AND NO I DID NOT JUST SAY MOBIL SUPPLIES ALL AMSOIL'S BASE OIL!!!!!!!!!!!!!!!!!!!!!!
