# Differential Oil



## imfastrnu2 (Jun 23, 2015)

Hey everyone, I am experiencing the very familiar "diff whine" on my 05 GTO with 16k mile on it. Obviously it's out of warranty and I would like to ask a couple questions. I don't want to belabor an already very old topic so I'll try to keep this brief and too the point.

I read the thread about the differential making noise and the "fix" being to change the oil at regular intervals.

http://www.gtoforum.com/f3/growling-clunking-noise-coming-rear-14102/

My questions:
1. What oil has proven to be the best performer?
2. Should I add a friction modifier?

I know this issue has been covered over and over and over again, but many of the treads were very old; around the time the cars were new.

Has there been any known cases of differential failure under normal driving conditions (not beating on the car)? And if so, has this proven to be an expensive fix?

Thanks
Dave


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## svede1212 (Nov 1, 2005)

If it breaks finding a used one is the best option money-wise. Oil changes are a good idea but having it completely stop the whine is unlikely. FWIW it can have gear whine forever and not break. The best fluid IMHO is Torco RGO 85w-140 with 2 oz. type "F" friction modifier. DO NOT USE the Type "G" which is the normal for GM cars. Ours are different. The purpose of the friction modifier is to provide a controlled slip for the cones that act as our posi. Without modifier the rear end will pop and make noises when turning corners as the cones grab and break loose. The fussy "best" way is to put in fluid, add about 1 1/2 oz modifier, drive a while to mix and then do slow, hard turns in both directions and listen for popping. If it's fine you're done. If it still pops and clicks add a little more and repeat driving and then slow turns. You want just enough to make the cones barely slip without popping. Too much modifier can cause one wheel burn-outs. I've seen posts where people say to add more modifier to stop whine. Those posts are wrong. It does nothing for that, it is for controlled slippage with the proper amount.


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## Badkat (Aug 11, 2014)

svede1212 said:


> The best fluid IMHO is Torco RGO 85w-140 with 2 oz. type "F" friction modifier.


^+1
Torco by far works the best with the GTO rear ends. I tried Royal Purple, clunked worse than ever. Switched to Torco RGO, clunking from the rear end gone!

Some other posts and forums may say you don't need the friction modifier with the Torco RGO. Don't listen to them. Add the 2 ounces of Torco Type F friction modifier with the RGO differential fluid.

Edit: free advice - Torco RGO does not come with the convenient poor spout on the the bottle like Royal Purple has. I ended up pooring the RGO fluid into my empty Royal Purple differential fluid bottles, and put it in the differential that way. RGO is extremely thick, and it is hard to poor through a funnel into the differential.


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## imfastrnu2 (Jun 23, 2015)

Thank you for the information guys, I'm glad I asked. This weekend I'll get some Torco and some type f friction modifier and get it changed. Thanks again!!


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## Badkat (Aug 11, 2014)

imfastrnu2 said:


> Thank you for the information guys, I'm glad I asked. This weekend I'll get some Torco and some type f friction modifier and get it changed. Thanks again!!


No problem at all. Got questions, just ask. I did many of times and this forum has always pulled through.

Just make sure you drive the car for 10 or 15 minutes to get the rear end fluid hot before you change it. You want to to get as much as the old crap out as possible. Let us know how it worked.


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## imfastrnu2 (Jun 23, 2015)

*Thanks Again*



Badkat said:


> No problem at all. Got questions, just ask. I did many of times and this forum has always pulled through.
> 
> Just make sure you drive the car for 10 or 15 minutes to get the rear end fluid hot before you change it. You want to to get as much as the old crap out as possible. Let us know how it worked.


Alright, I'll be sure to do that. Again, thank you.
The noise bugs me but I guess I'll get used to it, as long as it doesn't come apart, that's my main concern. The dang things only got 16k miles on it! :banghead:

Last question just so I have this right. It takes two quarts of 85w140 and 2oz of Type "F" friction modifier, correct? Just making sure since I have to order it from Torco and want to get enough for a couple changes.


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## Badkat (Aug 11, 2014)

Yep, that is correct. Its actually like 1.7-1.8 quarts.......but yes, you need 2 quarts to do the change. You fill the rear end until it starts to pour out of the fill hole. Once it starts to pour or drip out the fill hole, your done. Put the 2 ounces of friction modifier in first. 

BTW, the recommended 2 ounces does not have to be exact. If you put a little more in so be it. I think the Torco Type F FM comes in a 5 ounce bottle, i put half in mine, just like i'm sure everyone else did who did the Torco gear oil switch. lol


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## svede1212 (Nov 1, 2005)

Badkat said:


> Yep, that is correct. Its actually like 1.7-1.8 quarts.......but yes, you need 2 quarts to do the change. You fill the rear end until it starts to pour out of the fill hole. Once it starts to pour or drip out the fill hole, your done. Put the 2 ounces of friction modifier in first.
> 
> BTW, the recommended 2 ounces does not have to be exact. If you put a little more in so be it. I think the Torco Type F FM comes in a 5 ounce bottle, i put half in mine, just like i'm sure everyone else did who did the Torco gear oil switch. lol


It does make a difference for the reasons I stated. Friction "modifier" makes it more slippery so the cones, which act like clutches, slightly slip when needed. You don't want too much slip or you'll get one wheel peel-outs. You want just enough to make it smooth.


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## imfastrnu2 (Jun 23, 2015)

Thanks guys. Just to be safe I'll put in just two ounces and no more. Much appreciated fellas.


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## Badkat (Aug 11, 2014)

svede has spoken. If he says be exact, BE EXACT. 

And no, I'm not changing mine again for the possibility I put 3 ounces in instead of 2, lol.


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## svede1212 (Nov 1, 2005)

The important thing is to understand what the modifier does. It is to increase slipperiness to allow the limited slip differential cones (which act like clutches) to slip. When you turn the axle on the outside rotates more than the one on the inside and the inside one needs to slip to account for that. The popping sound you hear if you don't have modifier is the cones for the inside axle breaking loose periodically when forced by the wheel. Too slippery and it's slipping when it's not supposed to making it not a "limited" differential and you can get one wheel peel-outs.


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## gtiplus (Feb 6, 2014)

I had the same questions, thanks guys for the information.


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## imfastrnu2 (Jun 23, 2015)

Okay, so oil change is done.
Exactly 2oz of Type F friction modifier
Almost 2 quarts of RGO 85W140 gear oil

Preliminary Results:
Immediately after changing the fluid I warmed the car up by taking it for a 10 minute drive. The results were generally positive. Before changing the fluid, the whining was quite noticeable at 40-60 miles per hour. Now the whining is barely heard and only in a small speed window, around 50 mph. Also before changing the fluid, a clunking could be heard, and felt, when accelerating away from a fairly brisk stop, at a Yield sign, for instance. After the oil change, there was no noticeable clunking at any point.

Lastly, limited slip is working as it's supposed to, no popping around corners and no one wheel peel. 

I'll drive the car for the next few days and see what happens. If anything changes, I'll report back. As for right now though, I'm pretty satisfied with the results.

Thanks to everyone who provided recommendations and feedback to this thread, it is greatly appreciated!


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## Badkat (Aug 11, 2014)

Glad to hear. Torco FTW!


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