Pontiac 400 oiling problem

[Grim]

Starting to see some weird stuff with oil pressure.

Motor is a Pontiac 400 with a WC code and 6X-4 (350 head) motor making it the High output W72 Manual Transmission motor. I am running a Manual.

Whats happening is once warm when I accelerate the oil pressure drops about 10 PSI, take foot off the gas and it goes up 10 PSI.

Normally I'd think right off the bat that it would be crank thrust bearings but touching the clutch to put some pressure against the crank does not make it change. I'm also seeing the oil pressure bounce the needle on the oil pressure gauge at steady cruising around 2600-2800 RPM. I'm running a Mechanical gauge and in fact its been changed recently to a new mechanical gauge so not a gauge issue.

Anybody seen this before and find it to be anything but a thrust bearing issue?



Could it be my transmission has some excessive end play on the input shaft and its pushing against the crank and why it doesn't change with the clutch lightly pressed? Its not popping out of gears and despite 200k on it (original to the car and I have full documentation and no mention of it being rebuilt) it feels tight and why I didn't rebuild it.