Quote Originally Posted by JAMAS View Post
....am I crazy...or just missing somthing. What does it matter if you add the fuel additive before or after an oil change? They dont mix.
Because particles from deposits that get broken up or dissolved in your intake passages when using a fuel system cleaner can end up in your crankcase via blowby since that mixture ends up in the combustion chamber/engine cylinder.

You can obviously add a fuel additive whenever you want, it just makes more sense to do so in your last tank full of gas before an oil change so that any deposits that get broken up get later removed when the oil filter is changed, and better to remove those deposits at that time rather than leave them in the filter to contaminate new oil.