72 318 Scamp. Low oil pressure 1 minute after startup.

I changed the oil in my Valiant before I got it running. I put an entire quart of Marvels Mystery Oil in the oil when I did it. After about a week of driving it, I changed it over to 10/30 synthetic. When I drained it, I got all kinda globbiness out of it. I know beyond a shadow of a doubt some of it came off the pickup screen. It was taking several seconds for the oil light to go out when started. Now it goes out almost immediately.