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Discussion Starter #1
A few years ago I replaced the GM fluid in my differential with Torco 85W140 and F type modifier per forum recommendation. The results were not satisfying. I have decided to go back to OEM fluids. Finding the correct info has been a daunting challenge. I was able to find the 75W140 #89021809, but not the additive #8021958. Finally I did a Google search and found Pace Performance (www.paceperformance.com) had that GM # on the shelf. I confirmed the number and the application, and placed my order. Two days later I had it in hand. Inside the box was a 4 oz bottle of Spicer 43161 packaged in a plastic bag with the GM # on it. I went to the Spicer website, and it confirmed their #43161 as an equivalent to the GM #89021958. I considered several other options, however the lack of compelling documentation, and the fact that several major fluid manufacturers have recommended to me that I follow GM guidelines, has convinced me to go back to the OEM fluids.

The Spicer # can be easily found at a number of different supplier outlets. I found it at Jeg's, Summit, Amazon, and a few others.
 

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What was not satisfying about the torco? With my constant pinion seal leaks I can tell you the GM fluids that were in the diff was much more noisier and had to go back to torco. Between the two GTO’s torco has been hands down the best for me.
 

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Discussion Starter #3
With the Torco and type F the clutch lock up and release was unacceptably harsh, along with a lot of grinding during the lock up and release.
 

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with the friction modifier, sometimes you have to simply add a little more. it should clear up those issues.

Motorcraft XL3 friction modifier is the same substance if not close to the original modifier, IIRC. so is torco "type F." They should all work fine, you may just have to vary the amount added to the fluid.
 

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Discussion Starter #5 (Edited)
I kept adding until I was up to over double the recommended amount. Either way, I have the new product in hand and will be doing the change out this week sometime. I used whichever brand F type it was that came with the Torco.
 

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Discussion Starter #6
I might point out here that I'm not trying to start a thread about the virtues of one brand over another over another over another....there are lots of those threads here already. My intent is to let people know that the GM friction modifier is available, albeit under the Spicer label.
 

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I tried Lucas and trans x friction modifier. Made noise. Did the recommended torco and friction modifier and it made it a bit quieter. Didn’t completely fix it. So I put another diff in with a fresh torco change. That fixed it.
 
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