Hey guys. I have a dilemma. I own a 2001 for f250 7.3 4x4 6 speed 201,000 on the clock. 1 year ago the bearings went out in my rear diff which is a sterling and requires 75-140 synthetic. Well the reputable shop I took I to said that the 75-140 is to slippery for the clutches to engage properly so he put in 85-90 conventional with 3 bottles of friction modifier in it. Now a year later I have to have a new clutch pack put in due to it now putting a whole lot of fines into the oil with a small piece of steel and a sliver also found on the magnet. Now I am a firm believer in putting the proper oil in but I took his word for it. What do you guys think about this? Also is this failure due to the wrong gear lube in the diff or just mileage? Also I have only tow with this truck 3 times. Thank you, Jeremy.