I have a 72 elku with 454 performance engine. When engine was built it maintained 50 lbs oil pressure. After breaking it in and spinning wheels and shifting gears for a couple weeks oil pressure dropped about 10 lbs at idle. I changed oil to 10w 30 (wrong oil) and motor would idle at 40. Couple of weeks running it kind of hard it is idling 15 psi at 900 rpm. When driving at 2000 rpm it will hold at 50 and at 2500 it is always 60 and never fluctuates. When you let off gas it drops quickly to around 10 or less when holding the brake. Changed to 20-50 oil and it pressure is up about 20 psi on idle. Why did it change that much in 400-500 miles. Why wouldpressure dropsical much when slowind down but never change at mid speeds.