Originally Posted by tundraotto
oil shear - will drop psi way before fuel dilution could - and if it was fuel dilution it would just keep going....oil shears a lot at first and then 'holds on' while slowly shearing more.
Nope, fuel dilution i the primary driver of viscosity loss, which can be readily observed with a drop in flash point via a UOA. There is a percentage of it that flashes off once the engine is run at temperature for a while. This was mentioned a while back by an employee at Castrol who indicated that most modern VII's are quite shear stable and that in their testing, fuel dilution was the biggest contributor to viscosity loss, despite most on here blaming shear.