It sounds like you have an internal leak. It wouldn't happen with the 351 but my 427 had a
oil passage cap come partially off creating irratic
oil pressure readings... (un?)fortunately the cap was finally forced loose and I had a huge drop in
oil pressure 55-60 psi to 30 psi at street cruising speed.
I never lost pressure and drove the car home (probably a bad idea) I pulled the pan to find a 7/8" freeze plug (oil plug) and clip had come out of the crank (427 steel cranks are hollow).
I decided I would at that point install a HP kit in my new oil oil pump, a blueprinted HV unit. The result should have been (I thought) about 85-90 psi cold and 65-70 hot.
What I got was 160 cold and 100 hot... Not kidding... It was obviously way too high but it's interesting. I had really good oil pressure before changing the pump. Rather than changing the pump I drained the 20/50 and put in 5/30 (both Redline
synthetic). The pressure went from 160 cold / 100 hot to 100 cold / 60 hot. It will still spike if I don't warm it up properly but once warmed up it will not generally go over 100 psi and "criuses" at 75-80 psi. The internal leak it seems had been present since initial start up.
This is still a lot of pressure and may (probably will) wear my cam gear / distributor over the long run. I've spoken to a good number of sources and other engineers about my "problem" and the consensus is I don't need the pressure, although the engineer from Melling told me their HP pump was designed to run at 120 psi +. The guy who originally built my engine was big on high pressure (back in the 70's).
It has allowed me to go to the 427 S/C 0-160 oil pressure gauge...
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