Have always used Amsoil 15W40 Diesel and Marine. Also Amsoil Gear Lube for the outdrive.

Ditto on the Amsoil 15W40 Diesel and Marine on my 2002 HP 500s.

I can say that my old 454 mags would not carry oil pressure with synthetics. Switched back to regular oil and pressure stays up.

I have always run Merc oil. I currently run the Merc synthetic blend in my 496's. Usually put around 40-45 hrs per season.

what!

Pretty easy to understand. Synthetic oil and my pressure would run 20 psi regular oil 40 psi. Can't say if the regular is better or worse for the engine, but sure would rather see 40 psi.

I've never seen an oil pressure drop with switching to synthetics, but I did have an issue when I was younger with a timing cover leak after switching to synthetics. I switched back to conventional and it sealed up.

Thanks for all the advice guys. I really appreciate it!!

I have converted many things to synthetic oil, some as part of our business. This is what I can tell you, IF your motors leak oil at all dont think of it under any circumsatnces. If they dont leak and dont burn oil its the first thing I would do. I have personally seen flat tappet motors make higher hp on the dyno with ultra slippery oils n(like sentinel with moly in it) (our factory stock 355 sbc picked up around 10-12 ft lbs tq at peak on chassis dyno in back to back testing). I have also seen cars run faster ets with all synthetic conversions, case in point I used to own a bone stock pontiac 400 350 HP 69 Pontiac GP that I raced quite a few times, it ran 15.01 to 15.11 depending on air, it NEVER broke into the 14s ever. Switched everything over and it ran a best of 14.87 , it also leaked like a sieve thru the old style crankshaft rope seal and every other seam on the motor that used to just leave a few drops (it was 22 years old at the time and had never been apart). MY big hp boat motor on the other hand didnt pick up any hp anywhere on dyno at over 1000 hp, we were expecting a measurable increase when our 260 hp circle trcack motor saw such good results!, FWIW, Smitty
Avoid Royal purple, seems to be garbage especially their gear oil vs Amsoil in all my testing.