Slant 6 oil pressure woes…

I recently had a '64,/6 here that I was able to get running while on my trailer before I separated the trans from the engine, but once I got them apart and the engine on a stand I took the pan off and it was full to a point where it was covering the pickup completely with a sludge thicker than mayo. I drained the oil from it before I pulled the pan and it was the thinness of gas but didn't smell like gas. What came out of the drain plug was definitely as black as tar.... Have no idea how long it was in there.