I use a A/C oilfilter PF35L. It has an synthetic element down around 10-12 microns, and I can get it at my local GM dealer. pn# 25013454. When I first start the motor for break-in, I put a Moroso screen on top, pn#23845. Its a 280 micron screen, so it only catches the really big stuff.
I used to have an Oberg inline filter set up, but it has limited capacity to catch junk. Often it would turn on the plugged up light on in a matter of minutes. Dis-assamble and clean and redo. Just a big oily hassle.
We do every thing in my shop. We have a 'clean room' for engine assembly, but we also fabricate steel,grind, weld, body work,paint,mold fiberglass,etc. Everything, no matter how clean you try to be ,,gets in the air,,, and then in the motor.
Its amazing how much shop towel lint and guard dog hair that 280 micron screen will catch.
RICK