My 1988 year 454 crusaders maintain 40 psi if I keep the RPM around 2300. When I cruise them at 3000 my starboard motor drops to 30 but the port is good. This started after the latest oil change. Maybe a bad bypass in the new oil filter? Any thoughts? The motors run fine and don't seem to be straining. No unusual noises. My dipstick is at the full mark at 6 quarts which seems low to me but don't want to overfill. Maybe my oil cooler on that motor is not cooling enough and the oil is running warmer?