Oil pressure should not effect the rear main seal.

Please correct me if I am wrong. Going front to rear. You first have the bearing surface. Then you have the wing that sheds the oil that leaks off the bearing. Then you have the seal.

Crankcase pressure can cause you trouble. I usually coat the outer part of the seal with silicone. Some caps have the little cutouts for the seal with the extra material that extends out under the cap. I have had to cut that material off on my engines. I also have rotated the seal to put the parting line in the cap like mentioned before.

I'm sure someone can chime in as to which engines used the funky cap and seal with the extra material.

Leon