Had that problem in a chevy 383 I built yrs. ago.
Changed from castrol to Mobil 1.
Suggest u **** can the syntec oil.
Try it before you do anything else.
I lost 20psi running that crap in my chevy.
I know it sounds nuts but try changing to 15w-50 mobil 1.
Bet that solves your problem.
http://photo.starblvd.net/zanko20