I am swapping my noisy diff from my car with one from my buddy's parts car (same year and ratio). The old gear lube is fairly thick and black and I would like to get it good and clean before installing and adding the synthetic lube. What is best to use for cleaning the old lube? I have heard not to use a solvent as it will leave deposits that could damage the gears. It was also suggested that I use transmission fluid to clean it as it is fairly thin and viscous, but I am not sure how well it will clean the old lube. Thoughts?