Not finding rear axle capacity in Service Handbook (Capacities, pg 2-3) or Approved Consumables (ibid, pg 3-3) I searched on-line and found 2.6 L (2 coolers), or 3.0 L (3 coolers). Unsatisfied and keen to "upgrade" from Castrol SAF- AG4 to a suitable GL-5 product, I asked my local "Indy" to drain and re-fill Final Drive with Red Line 75W90 Full Synthetic GL-5 High Performance Lubricant for Hypoid and Limited Slip Differentials ("Extra friction modifiers are included for use with limited-slip differentials (additional friction modifier additive not required unless chatter occurs.")) I brought 3qts (p/n 57904). Mechanic advised that diff took "a little less than 2qts (full to run-out) and returned 1qt. SO, call it 1 qt, 24 oz, or 1 cup short of 2qts "full to run-out." Downshifts remain smooth and silent, as before service.
Ed Trottier
Aiken/Beaufort, SC

 

Hello Ed, @trottier.ed
I think you may have some confusion as to the differentials in your GT, it is either 1 cooler or 2 coolers, not 2 or 3, and these coolers only have to do with the front and center differentials, as the rear differential that you are referring to does not have a cooler, nor does it hold anywhere near 2 quarts, it holds 1 liter or 2.1 US pints, it is not a fill to flow out level filling procedure.

I would drain and refill with the proper amount.

See attached screenshots.

Johnny

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Thanks for your kind and informative reply. Yep, got my 2's and 3's mixed up, but it is moot nonetheless. No mention anywhere about "sealed for life" rear diff (Service Fill is stated, and it has a HIGH COST quote for it. Odd.) Bentley Charlotte or Atlanta told me quite clearly, "It is a 100,000 mile Service." Odd. Perhaps "Repair/replacement Fill" would be more suitable. Given car has less than 50k mi and strides made in lubricants, I figured time was right for "upgrade" to GL-5. Wonder what the failure mode is for 1L versus maybe 1.5L with a "breather" port. "Foaming?" Odd that no "over-fill fail-safe" with overflow above my "first try" 1L GL-5 volume. I would believe different fill volumes given 1 or 2 coolers (unless there were some very clever design/engineering/location adjustment(s), but this looks like a rabbit hole. I'm gonna call Bentley Charlotte, who is quite proud of their (1 of 60-some) Bentley Master Mechanic, maybe chat him up for advice. In the meantime, be assured I appreciate your time, knowledge and effort. Would appreciate your assistance/advice in solving what looks like a much larger, systemic Bentley Driver information problem here.

 

Not foaming as much as submerging the axle seals which can cause early failure. The design is usually so the level sits right below the seal bottoms.

 

​​​​​​Hello @trottier.ed ,
I just want to be clear, as you mentioned the 1 or 2 coolers again, the coolers have nothing to do with the final drive (rear differential) as it has no cooler, the 1 or 2 coolers are for the front and center differentials, so the rear differential volume is the only volume.

As far as what damage could happen, well, I am not 100% on this but, I was driving on my first long drive from NJ to Newport Rhode Island, and I heard a rear differential whine when on and off the throttle after some miles driven, kind of like when the ring and pinion are not set up right, and being that the vehicle only had between 9,000-10,000 mile on it, I honestly didn't give checking the level of the fluid prior to my drive much thought, but what I did do first, was to loosen the drain plug to see if I had any water contamination, the oil was clean and no sign of water, so I moved on to the next item on my list before the drive, well the whine bothered me so much that I thought the differential may explode, as I thought maybe it was dry, did someone else drain it, as I only tested a small amount coming out, well long story short, I did just as you did, I purchased 3 quarts and found a place to drain and re-fill the rear, and when only a quart of fluid came out, I was pissed at myself for not checking for the proper fill before the road trip, well I filled it to flow out just as your guy did, and I still had that slight whine, and thought that I did damage by not having it filled to the proper level, only later to find out when I returned home and checked the info, that the level was correct, and now I over filled it, and now I have slight seepage around the pinion seal that was not there before I over filled it, and I still have the whine, but I have a whole spare rear subframe assembly, so I may just swap out the diff...

Good luck !

Johnny

 

​​​​​​Hello @BWings ,
Now you tell me.... 🤣🤤🤣

Makes sense.

Johnny

 

aHA...axle seals! I will get right on this and drop level to 1qt. Thanks for the info.

 

Sorry, wrong "word". Take the word axle out, just "seals" in the case. Both input and output.