The majority of Buick Grand National, Regal Turbo T’s & T-types, etc., all have 8.5″ positraction rear ends in them.
[Regal base models may have 7.5″ open differential units, meaning non-posi rear ends. These open rear ends do NOT require any additives other than gear oil]
[note that “positraction” is the GM term for what other brands call a Limited Slip Differential, of which may be referred to as a “LSD”]
Every so often, per the manufacturers recommendation, the fluid within the rear differential needs to be changed.
Whether you call it rear end fluid, rear end oil, axle gear oil, axle fluid, differential oil, etc., all of these different terms mean exactly the same thing. This is the stuff that lubes up the parts back there to make sure you keep moving forward (and occasionally backward!).
We covered how to change rear end fluid on here previously, in case you need step by step directions on what is exactly required to perform this task.
We did a lot of research to determine what exact fluid is recommended, by actual Turbo Regal owners, as well as the makers of the positraction units themselves.
SYNTHETIC OIL TALK:
Auburn as well as Eaton (manufacturers), include a note with brand new units that say: “Do not use synthetics, it will not allow the clutches to work properly.”
[one Buick owner stated he believes Eaton supplied the OEM posi to Buick]
There’s much debate over using synthetic fluids in the rear differential (as well as in other areas of the vehicle).
Cost is just one example. (one bottle is about $20)
75-90 is the usual weight and choice for synthetic oils.
For lots of Buick Regal race cars, their owners say they prefer using synthetic oil.
(but of course, it gets changed way more often than regular daily driven or show going only type cars)
Some drivers experience chatter when using a synthetic fluid.
Chatter, or posi noise, is hearing a noise at a very low speed, usually heard in tight turns, or when going around a corner. It will sound like something is moaning, and will feel like something is grinding or rubbing.
The “cure” for chatter, is adding posi lube (AKA “limited slip additive”) to the rear end.
One (4 ounce) bottle is usually enough.
(but we have heard others say that they needed 2 bottles to fix their issues)
You should be aware however, (when using synthetic fluids), that posi lube should only be used if there is an issue with “chattering” when turning, as it does REDUCE the efficiency of the posi-traction unit by letting it slip more (that’s how the additive works).
Some Turbo Buick owners have used Royal Purple (a synthetic version) with NO (Limited Slip) additive.
Mobil 1 synthetic gear oil is another option, and it already has limited slip additive included.
CONVENTIONAL OIL TALK:
75w95 or 80w90 is the usual weight and choice for conventional oils.
ANY quality brand will do.
(Valvoline is probably the best known type)
(it runs about $10 for a quart)
The price for the conventional oil is much more reasonable than synthetic version.
(about $10/bottle versus around $20, even though total quantity needed for a standard rear end is usually only around 2 bottles, perhaps just a bit more)
The majority of Turbo Regal owners prefer this type of axle gear oil.
Some call it “dino fluid,” whereas dino means dinosaur (the tried and true, old oils).
The overall general consensus says go with the dino (conventional) oil plus the GM additive.
(There’s other brands of additives available, but why use anything else when the cost of the GM AC Delco stuff isn’t that much, and is actually MADE for the Buicks)
You never (usually) hear of problems by using the factory products (conventional oil + GM limited slip additive), so that should be a clue for most Buick Regal owners.
The costs when changing out your rear differential oil (using the conventional stuff) is approximately $20 for the gear oil, $15 for the limited slip additive, and $10 for a gasket, bringing your total to somewhere near $45.
[and most Regal Gbody owners will most likely only do this once while the car is in their possession]