I own a 1948 Chevy truck with the original 216 engine. I was told the engine was completely rebuilt in 1985 and once finished, the truck just set until I got it a little over a year ago. When I received the truck the first thing I did was change all the fluids including the oil. Not knowing about 216 engines, I replaced the oil with 10-40 detergent. The truck has run great and the oil pressure runs about 6-8 pounds at idle and 15-16 while driving. This weekend I did maintenance again changing the oil only now using 30w non detergent as was suggested. After I finished, the oil pressure now shows 15 pounds at idle and pegs out at 30 pounds revving up. Now I didn't drive it due to the heavy rain. Any reasons why the drastic change in oil pressure?