Any easy way to get it in? Even with a very light amount of oil around the edges, it's exceptionally prone to slipping in sideways and then it won't seat at all.

InfO.

My service manual says to use a socket and hammer... but I'll be damned if I have a socket large enough as the seal.

 

Use a piece of 2x4 wood to help set it in, also that seal does not sit flush with the case, there is a lip cut in the case, seat the seal to it only.