Six Speed, I monitored oil pressure with 10W-30 more closely yesterday, and made a better post of the difference. Here is what I posted earlier at Team Shelby.
The 5W-50 is there to cover Ford's butt. The car stock is capable of massive heat soak and high oil temps, so I can understand the 5W-50 spec. Ford also knows the oil will shear easily to a heavy 30 grade lubricant, which they must still be comfortable with, or it wouldn't carry a Ford recommended 7,500 mile oil change interval using it.
The car with a modified cooling system (radiator and intercooler) is a complete different animal. Oil pressure with heavier oil was excessive and wasn't required. Even in 90*F+ summer heat...10W-30 is in harmony with my modified GT500.
I see an average of 5-10 PSI lower oil pressure using 10W-30 versus 10W-40.
Amsoil 10W-30: Cold start 65-70 PSI, Hot idle 25 PSI, cruising @ 1500 rpm 57-59 PSI, cruising @ 2000 rpm 70 PSI (yet to establish what rpm the bypass opens)
Amsoil 10W-40: Cold start 75-80 PSI, Hot idle 30 PSI, cruising @ 1500 rpm 65-70 PSI, cruising @ 2000 rpm 75 PSI (bypass is open by 3,000 rpm)
Motorcraft 5W-50: No testing performed, but theory would say it's the equivalent to 10W-40 after some shearing.
A true synthetic 10W-30 is still plenty of oil pressure for street use, aggressive street driving, and short burst drag racing.
