I had that issue on one of my engines - start up had good oil pressure - 80 then it fell at idle to 10 pounds, came up and stayed at 70-80 down the track.

I pulled the intake on the engine stand and drained the oil and installed clean diesel fuel and primed the engine to simulate hot thin oil.

I had a torrential rainstorm going on - seems the roller lifters had the lightening groove a tad too low when on the base circle and when originally priming the engine during the build I missed this. Used a set of old Engles and problem fixed.

It amazes me that the manufacturers can't / won't develop a design that allows lightening the lifter without offering this risk for those that don't bush their bores.

Maybe they do now and I haven't seen them.