Comes with doing millions of diffs, there's no insight, just be methodical and chase out the gremlins trying to understand what is going on rather than follow a quick formula (use this or that miracle ointment).
The centrifugal force will throw out the oil no matter what but that doesn't mean the diff has to leak. The gaskets in the kits are very good, but you need to be careful not to damage them when taking out the cutout holes or whatnot. I soak mine in a bit of oil to make them a bit more flexible and like I said before, I sand the mating faces of the diff halves. Be realistic, these are not swiss watches we're playing with here. They're a cheap (expensive) toy car in a kit. There are imperfections and such. Take a good look and you'll see. Sometimes the gasket is not pressed enough because the diff halves can not come close enough to each other to press it. Look and decide how much you need to sand. I've seen this even on Xray diffs which I rate as amongst the best as far as quality of execution goes. Functional quality is different, but even there the Xray is up there at the top (there's always room for improvement).