Like you I started with AN-4 and now I'm at AN-8. Seems to let the oil flow out much better, but a sump resevoir would help even more. I just can't figure out where to put one without it being too low. I'm doing a 25 psi check valve on the feed side and may go to 0.010 restrictors.
Edited: Just to be clear it's AN-8 on the return side. Still AN-4 on supply side.
EFR Twin Turbo LFX-GPI Tune-ZL1 fuel pump-10:1 CR forged pistons-3.45 gear-Meth Injection-BMR Trailing Arms, Bushings & Sway Bars-CircleD 4000 Stall-GPI Fuel Enrichment System
647 RWHP & 726 RWTQ @18.5 psi on 93 Octane (locked converter)
1/8 mile -- 7.158 @ 102.10 (20psi); old build