Ed,
After break-in, use the lightest weight that gives good
oil pressure. Lighter weight
oil is easier for the engine to pump around and if your clearances are tight enough will give you the most power and the best efficiency.
My engine was built pretty loose. After break-in, I tried 10-30
synthetic and the
oil pressure at idle, hot, dropped under 20 psi. I switched up to 20-50 and the pressure is fine. Now, I'm trying Castrol
Synthetic 10-50. It is OK most of the time, but when I'm idling a long time (in traffic), the pressure gets a little low. I'll go back to 20-50 Mobil One next time.
Paul