Just a note here: I know a gentleman, now retired, who was a professional lubrication engineer and who consulted on the additive package for the first generation of synthetic oils.

He runs 25,000 mile oil changes when using a quality synth. Yes, the comma is in the correct location. Twenty-five thousand miles.

What he does change, every 4000 miles, is the *filter*. Filter media gets soggy and saturated, he says, mostly with carbon crud from ring blowby and the cylinder walls, and the filter isn't good for much more than that.

I think he's nuts. I change synth and filter both at 6K. All of our engines are quite clean, and oil analysis shows no issues.

s/b