My only experience with a 4.0 rear main was horrible. The first try I accidentally put a small nick in the seal when installing it. But I kept on going anyways and sure enough, it leaked worse than before. So I did it again. And once again it leaked. So I did some more research and noticed that the repair manual called for RTV in certain places around the seal. I'm not sure why I didn't notice that the first time, but I tried again with the silicone. It STILL leaked. Now I was getting pissed but figured that I didn't leave enough time for the RTV to cure. So I did it one more time, used lots of RTV (be careful, as you don't want chunks to break off and get into the engine), and waited overnight. That apparently did it for me and I haven't had any issues since. :D Well, with the rear main at least. Later on I fixed the valve cover, but I still haven't bothered with the oil filter adapter's seals. They leak pretty bad.