Hi Ray, they don't recommend Synthetci for break in, as it usually won't seat the rings as well since it inhibits the friction etc...

After break in, maybe even after the first season, I would then switch. Better yet, contact Amsoil, and or search on their website, I'm sure they have the answer there!

It's excellent on High Milage cars! You get improved power and milage, and it will last even longer.. or many more miles that is!

Hope this helps - Brett - 67 Drop Top 455!