I used marine grade wheel bearing grease in the boot (just because that was what I already had in the grease gun) with a hypodermic (sp?) needle attachment you can get at a local parts store. I pumped in a bunch - dont remember how much. maybe 10 pumps? drove a bit and it was gone for a while, so I did it again and haven't had the clunk since. I dont think you want to fill the entire boot, but enough to lubricate the splines. After road use it probably works into the shaft.