Remember if you stick Mobil 1 in the diff you should change it again in a year. When I first did mine the fluid was probably original and came out looking real clean. Just for the heck of it I did it again a couple of months ago while the car was blocked up with the trans out. The one year old Mobil came out black as engine oil! People are not kidding when they say that stuff cleans!

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2005 Chevy Silverado, white on black.

"Posterity! You will never know how much it cost the present Generation to preserve your Freedom! I hope you will make good use of it. If you do not, I shall repent in Heaven, that I ever took half the Pains to preserve it."
— John Adams