I disagree with Jim on this one. Over a period of years, moisture can build up in the gear case, and with friction on the gears, the lubrication tends to break down and turn black, etc. I changed my rear end lube to a 75-90W synthetic lube, and I also changed my standard transmission with the same stuff. To me, it is just good insurance that the lube in the rear and tranny are fresh and the latest product that will protect them. It was like $65.00 to do that. Cheap insurance.