Had a buddy that had the same problem with his 413 and couldn't figure out what was going on, he brought it to me in the hopes I could fix it with out tearing it down, the car would lose oil pressure but not knock and would run fine. Started looking, pulled valve covers and intake and found that he went alittle over board with RTV sealer and some got down into the oil pressure passages in the block, so what we did was what Mr P said put lines in a 5gal bucket and started it and blew out all the crap that was in there.