i usually spray wd-40 in there and turn the diff while holding a rag in the gears, works great and the little shims don't get lost. also before you put the grease in the diff be sure to check the mesh by tightening 2 screws across from each other and turn the diff cup slowly. the opposite side should spin the opposite way smooth with no stopping and just a tad bit of buzz of the gears teeth touching, if its too tight add another gasket, if they are clicking and sticking you need to shim under the cross pin and try again. diffs expand under heat and can separate the gears just enough to round them off and eventually grind them down to nothing. i've gone through 5 8th scale cars and diff issues and this way has worked flawlessly.