What is 75W140 for ??
Is it the gears that need it ???
Or is it the LS clutch disks ??

 

It is the recommended weight oil for your differential, and is synthetic in nature. If you have a tag on your housing stating 75w140, then you need to run that gear oil for proper operation and longevity of your axle as recommended by Ford.

Robbie

 

It is as bigbronc said "It is the recommended weight oil for your differential, and is synthetic in nature." Additionally if you either adding the 75W140 or totally replacing it and you have a Limited Slip Differential then you will either need to get the 75W140 with the LS additive already incorporated into it or purchase the LS additive seperate. If you purchase the LS additive seperate it is approximatly 1 oz additive to each qt of 75W140. The smallest additive bottle I have found is 4 oz.