I have to admit I'm a recent, tentative convert. As can be seen below, all my vehicles have been switched to synthetics. I've done a couple of UOAs, one on the infamous Mazda 2.3L turbo DISI and another on the controversial Honda VCM V6. The Mazda is a proven fuel diluter. PP after 3300 miles had 2.8% but 5+ TBN left (and very low wear metals). How far do I push OCs when I know there is dilution? I'm not sure I want to discover what is too far.
The Honda has an MM and I obeyed the dealer and the manual leaving the FF in for 6300 miles, against my better instincts. The oil had 2.0 TBN when I changed it but 250+ppm Cu. How can it be good for any engine to have all those metallic wear particles circulating?
It seems to me that one does no harm EVER changing early but that harm can certainly be done by not changing soon enough. If I had engines known to be easy on oil (like I hope the Toyota will be), I can get comfortable with their 10,000 mile OCI. Will the FF stay in that long? Not after seeing that Honda UOA at 6300.
The well used Mopar V6s? I have no problem putting M1 in there and seeing them in a year.