Here's the thing. The later weight is the at op temp. So when you stop the engine and it's cooling down that high weight viscosity is also cooling down at a temp rate equal to engine temp. Higher temp weights adhere to valves and moving parts longer during cool down and can create added deposit adhesion to these parts. At least that's what a mechanic,race car builder, driver explained to me when I told him I wanted to change to higher viscosity rating because of higher mileage.

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X5 3.0 AUTO -2001 - 238,000 miles