A few emails about replacing the lube in your differentials and transfer case with transmission fluid. This should only be done if you vehicle is designed for lighter fluid use, or you could cause some very expensive to repair damage, 80-90 is still the fluid of choice by most pickup manufactures. Synthetic is the way to go with a total fluid replacement, more expensive but worth it. I have always been impressed with how quiet the drive line is after a synthetic change.Do not overfill the diff or trans as this will cause foaming and again will cause some lubrication problems resulting in repairs. This is a common mistake, a little more will not lubricate more efficiently, this is not the case and the same is true for engine oil. PS Several questions on why the synthetics quiets the drive line when you would think that is is thinner it would be nosier?First it just plain is a more efficient lubricant.It also clings to the gears as they pass out of the lube so they are lubricated as they make first contact on the way down to the lubricant level.