Hey John:
I had a similar problem with the 351 built by installed by HRE. I drained the 50W-90 that was in the crankcase and replaced it with 5W-30 Mobil1 and it went away! I also got rid of the Fram and put on a Mobil1 filter. I'm convinced the thinner
oil goes through all the plumbing more easily. Just my $0.01 worth.
Bob