I'm wondering if people have seen a reduction in oil pressure when switching from flat-tappet lifters (hyd or solid) to solid roller lifters in OEM big blocks w/ unbushed lifter bores.

FWIW, I'm going to be using the roller lifters that Brian @ IMM used to offer, which have a cut-out in the middle of the body comparable to the COMP 892s. Here's a picture of the IMM lifter, although mine have the offset intake pushrod seat.