I used to give syn oil. Now I just found out my dad has been running regular. He says the manual calles for oil changes every 6,500 miles. Is it O.K. for a car used to synthetic every 3K to run on regular for 6.5K? NA 93 BTW.

i wouldn't do that on dino oil regardless.

Engines don't really get "used to" oil. You can go back and forth fine.

Isn't it true that if you have an old engine, switching to synthetic is a bad idea because it might be leak prone?

Glad to hear that. May explain why dad'* engine hasn't failed yet.

What i've heard (makes sense) is that regular oil can gunk up holes so you don't get leaks. The extra detergents in Syn cleans out your oil (why it gets dark quicker on your first couple changes) and doesn't leave deposits. It also flows a lil better, especially when cold.

Whether or not a car will be safe depends on how clean it is in the oil passages. I switched @ 112, and i'm good. My oil pan gasket might drip every now and then though. :?

i thought that switching from regular to synthetic back and forth will mix them up in your engine and make sludge.