I know that you guys are not going to believe me, but I've never put any sealer between the rear main cap and the block and I've never had any issues with leaks. I use real fine sandpaper to clean out the groove in the main cap and the block and then I throughly clean both grooves out. Cleanliness is the key and to not get in a hurry. I also check the crankshaft for burrs where the seal will ride against. Before I install the seal I check it out for any nicks as I've had a couple that where bad from the factory. I offset the ends of the seal and I put just a dab of sealer on the ends of the seal. By me doing it this way I can honestly say that I've never had a rear main leak, luck, maybe so.