You do have an aftermarket oil pressure gauge on the car right? If not, put one on. Then use the thinnest oil you can, while maintaining the oil pressure you need. You want 11-12 psi/1000 RPM. That means if your full throttle shift points are 5000, you want to see at least 60 psi at 5000 RPM. If you can get that with 10W/30, that's what you use. If not go heavier on the oil until you have adequate oil pressure. If you don't have good oil pressure and you rev the motor, or it gets too hot, you'll have rod bearing problems. (#7 & 8 are the ones that usually go.
I use 10W30. I have about 15-20 lbs. at an idle, @50-60 driving around and I peg the factory gauge at WOT.
I haven't been real happy with my hot idle oil pressure, been using 10W30. Sometimes would drop to under 10psi. I changed to 20W50 yesterday, and my lowest hot idle pressure has been around 20. I don't typically start it up unless temps are 50deg +, so it should be ok for me. 10W40 might have been a good choice too.
Thin is good, especially if running an HV/HP pump (most will tell you such a pump is a mistake - but I diverge). Running 5W-30 synth with one of these "bad" pumps. Larry's advice is right in there. :TU: With a stock pump in the summer 10w-30 should be fine and don't use race oil unless you happen to like rod-knock at start-up.
20w50 i run 20w50 oil in my 455. my motor has 141,000 miles on it and i maintain 17psi at idle and 58 @ 5200rpms Dustin
