BBC oil pressure?

[Slammed70]

Workin on a 97 1/2T, its got a transplanted mid '70s 454 in it.

When you first start it, oil pressure is good.....3/4 of the guage and damn near maxed with highway RPMs.

But after about 15 mins or however long it takes to warm up, the oil pressure dramitically drops. wont even register at idle, barely moves/flickers at RPM.

Any idea what would cause this.

I thinks its got 10-30 Mobil 1 in it, other than that I have no idea.

Thanks,
Mitch-