3% soot is the practical limit. I would never change syn Delvac after 3600 miles unless there was something wrong with my engine. Normally 7500 is safe is you do a fair amount of highway driving. I've gone 9500 and everything is still fine. After 94000 miles of syn fluids averaging 7000 between drains my timing chain had zero degrees of stretch.

HTH,

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Brian Toscano