Fastlane parts;

[RC-Monster Mike]

Even the thick oil will leak out. Slowly, but it will leak out a little. Some racers use the heavy oil without the o-ring seals. They just build before the race and it works. I am a low maintenance kind of guy, myself, though(too lazy or too busy to spend time building diffs all the time - I like the Ron Popeil strategy - Set it and forget it!). I just use a small amount of lithium grease on the ring and pinion gears (I think this is what you mean). A heavy oil would work fine as well.