I think I would have good piece of mind with the 40 grade vs the 30. How much do you think would be gained going from 50 to 30 grade oil. Would really like to know hp differences between the 2. Current oil pressures are 80 cold, 30 hot at idle and 65 hot WOT.
Try what you are comfortable with trying and see how it effects the pressure especially at the high rpms.
There's a good chance that idle psi drops a little and high rpm pressure increases going to a 15W-40. (when compared with 15W-50 at the same oil temperature)
If so, then next change you can try 10W-30 if you want.
But if the high rpm pressure drops then of course you don't want to go thinner.
