For the rear diff, use any quality hypoid (GL-5) lube you wish. (synthetic will improve MPG)
For the manual-Xmission/front diff (shared lube) It is HIGHLY recommended to use the
Subaru EXTRA-S lubricant.
This is because of the 'shared' nature of the lubricant.
- The differential gears need hypoid lubricant (GL-5)
- The Xmission needs additives for the synchronizers to work properly (GL-4)
For many gear-oils, these two properties DO NOT come in the same lubricant.
- The sulfur/phosphorous additives for hypoid Extreme Pressure (GL-5) can wear down the synchronizers.
- The additives which aid the synchronizers to change gears smoothly (GL-4) need some 'grip' in the lubricant....which hypoid gears do not like.
Hence, Subaru has blended their "EXTRA-S" lubricant to meet BOTH of these demands within a single gearcase. It is not cheep... but you should only have to change it ONE TIME during the life of the vehicle.
Some folks have used other lubricants within the manual-Xmission/front diff... but the 'feel' of the shifting may get 'squishy' or 'notchy'. Also, gear-clash may be experienced on cold mornings.
If you are the kind of driver who does not care about the 'feel' of the shifting, nor about wearing out your synchronizers... use whatever lubricant you wish.
