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Old 06-04-2013, 09:24 PM   #1
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Post 1998 z28 differential.

I want to change the fluid in my 98 z28, however I keep reading conflicting opinions on what fluid to use..

I have read that synthetic fluid is not recommended, others say only use synthetic. The only solid information ive came across for this year diff is that the car needs roughly 2 bottles of fluid and a bottle of GM brand additive of lsd fluid.

The owners manual calls for 75-90w synthetic fluid, but since ive read otherwise im trying to make sure if this is in fact the right fluid and weight.

When installing a new gasket should I use a thin bead of rtv on both sides of the gasket?

I have never changed diff fluid.. Im hoping someone can point me in the right direction.

Thanks for any input in advance!
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Old 06-05-2013, 11:09 AM   #2
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I also have a 98 and used 2 qts of regular 75-90 fluid with 1 4oz bottle of additive. When I put the cover on i didnt even bother with the gasket, I just put rvt around the whole thing and it never leaked the 2 times i changed it. word of advice, get a hand pump from a parts store for putting the new fluid in.
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Old 06-05-2013, 01:05 PM   #3
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I also have a 98 and used 2 qts of regular 75-95 fluid with 1 4oz bottle of additive. When I put the cover on i didnt even bother with the gasket, I just put rvt around the whole thing and it never leaked the 2 times i changed it. word of advice, get a hand pump from a parts store for putting the new fluid in.
Thanks.
Do you know for sure what diff is in your car?
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Old 06-05-2013, 02:11 PM   #4
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I have an auburn. all 99+ cars have a torsen.
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Old 06-06-2013, 03:09 AM   #5
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Syn has the additive already in it. Dino doesn't so you'd need to add it. 75/90 2 quarts. Simple.
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Old 06-06-2013, 11:00 AM   #6
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I just changed mine in 2000 last week using Mobil1 oil. I was curious about if I needed the additive or not too, but from what I read it won't hurt anything to add it, so why not?

I oddly didn't need a pump to fill mine, just held the bottle up and squeezed. Which was VERY surprising because I just changed the rear end oil, front end oil and transfer case fluid on my 02 Silverado and needed a pump for all of them.
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Old 06-06-2013, 12:01 PM   #7
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the quickest way to check what differential you have is jack the rear up with both tires off the ground. spin one wheel, if the other wheel spins the opposite direction you have the torsen. if it spins the same direction you have the auburn.

the owner's manual is somewhat incorrect or doesn't explain the whole story.
the torsen differential does not have clutches, so it does not require limited slip additive. but torsen recommends using it to keep it quiet under slow speed turns because it can make what was called a rusty screw noise which was normal and did not hurt anything. the auburn diff uses clutches and does require a limited slip additive otherwise in tight turns the clutches release and slip/stick causing a chatter. limited slip additive, also known as friction modifier, allows the clutches to smoothly slip when turning. in either case you want to use as little additive as possible. using too much can reduce the oil's lubricating ability. the amount was something small like one ounce per quart, and when you buy the additive it will be in a small bottle or tube. you can always had an ounce more later, not adding enough to begin with will never hurt anything. and you don't have to buy GM brand additive, you can get generic at any parts store. also the auburn differential if you have it has always recommended conventional mineral GL-5 gear oil and not synthetic. whether you use synthetic won't damage anything but auburn has stated that the diff performs best/smoothest with mineral gear oil 80w90. so unless you are endurance racing or towing with your car and knowing the differential oil is getting very hot, buying synthetic is a waste of money and many times will end up leaking past the pinion seal.
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Old 06-08-2013, 03:17 AM   #8
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Quote:
Originally Posted by 1 FMF View Post
the quickest way to check what differential you have is jack the rear up with both tires off the ground. spin one wheel, if the other wheel spins the opposite direction you have the torsen. if it spins the same direction you have the auburn.

the owner's manual is somewhat incorrect or doesn't explain the whole story.
the torsen differential does not have clutches, so it does not require limited slip additive. but torsen recommends using it to keep it quiet under slow speed turns because it can make what was called a rusty screw noise which was normal and did not hurt anything. the auburn diff uses clutches and does require a limited slip additive otherwise in tight turns the clutches release and slip/stick causing a chatter. limited slip additive, also known as friction modifier, allows the clutches to smoothly slip when turning. in either case you want to use as little additive as possible. using too much can reduce the oil's lubricating ability. the amount was something small like one ounce per quart, and when you buy the additive it will be in a small bottle or tube. you can always had an ounce more later, not adding enough to begin with will never hurt anything. and you don't have to buy GM brand additive, you can get generic at any parts store. also the auburn differential if you have it has always recommended conventional mineral GL-5 gear oil and not synthetic. whether you use synthetic won't damage anything but auburn has stated that the diff performs best/smoothest with mineral gear oil 80w90. so unless you are endurance racing or towing with your car and knowing the differential oil is getting very hot, buying synthetic is a waste of money and many times will end up leaking past the pinion seal.
Thank you very much for the reply!

However when i went to turn my wheels "spin one wheel, if the other wheel spins the opposite direction you have the torsen."
It spins like i have a torsen, witch wouldnt make sense to me, from reading what you said and other postings. Did some 98 cars include torsens?? Im really confused now..

However I put in castrol 80w-90 roughly 1.5-1.80 qt and about 3 of the 4 oz of the gm additive.

It seems to run fine right now but I havent drove more than 20miles, should I drain and redo it or just run whats in there now?


Thanks for any insight!!
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Old 06-08-2013, 03:17 AM
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