I managed to come across a Motorguard oil filter. It has a metal mesh in the top half although I can't see what it was for. After removing the mesh a standard toilet paper roll presses over the bulge in the center tube top half nicely. However there is no such fitting in the bottom half, it would be relying on the raised ring to block oil flow which I have doubts about. I think I have seen someone else using an O-ring here, any idea what size?
I did find what seems to be the right gasket:https://www.amazon.com/gp/product/B0051XZM54/
I have some questions about the incarnations of Motorguard oil filters. The version I got has the crosshatching lines in the upper half ceiling rather than radial. Both forms seem to be different from what is in this picture:
Should I assume that Motorguard filters don't really fit most toilet paper rolls regardless of what vintage?
At first I didn't see how the oil would flow into the center tube at the top but after looking at some pictures I realized the top tube was probably pressed into the outer shell, leaving gaps for the oil to flow through. I put a flashlight into the shell and could see light behind the snap ring on the clamp nut.
Concerning oil flow, I am under the impression that there is a pressure valve in the oil pump in most cars that drains excess oil back into the crankcase, maintaining the post-pump pressure at some valve depending on oil thickness and RPM. So logically, the oil loss through the Motorguard will just be oil that is no longer flowing through the pressure valve, and the oil pressure should be about the same.
It seems to be pretty hard to find 4.5" toilet paper these days. This motorguard came with what I assume is a M-723 filter in it. I don't think it was ever used. There are no toolmarks on the bolts or mount and it's bone dry, with a light layer of rust on the center bolt.