360 oiling problem under load

An oil filter would normally collapse in neutral and not moving, just as much as moving with engine under load. I would be looking at the pickup system as suggested: either in the tube or the pickup head or something in the pan moving under acceleration.

OP, also measure the quantity of oil that you drain very carefully. Your dipstick markings could be off and misleading you; there are several dipsticks out there for different pans and engines. Also, if the pickup is too high above the pan, it could becoming exposed to air if you burned up some oil since your last change.... which sounds like it was 2500 miles ago from your info, so you could have burned some off and have lower oil than when you last filled it. In fact, before pulling the pan, I would add half or 3/4 of a quart first and see if that fixed things.....that would tell that part of the story.