rear diff fluid

[oldman14ss]

what fluid do you folks use in your truetrac ? I'm breaking mine in right now looks from my mechanics pic its running a little to the toe of the gear at 5 thousandths back lash. like another poster mine makes a load whine on the coast side around 60 to 65 mph. have 120 miles on break in. it is starting to quiet down some. broke it up in 5 driving sequences, heat cycles , back roads rowing the gears so as not at a steady speed. have 75/90 lucas oil gear lube what eaton recommends. I figure I get 250 miles on it. then cruise at steady highway speeds. 3:70 aam gears. anyone go against the grain and run 90/140 in your true trac? whats a good one to control whine if I have to. thoughts? suggestions? the oldman.........

[Moto-Mojo]

I’m stock but I just used Amsoil in the squeeze bottle. Probably didn’t have to put in the LSD additive but I did. Used Redline for that. I forget the exact weight oil but I used what the manual calls for. No issues.

[fz4k98]

I'm staying with the Lucas product they had me break it in with. Changed it at 500 miles to get rid of any break in sludge.

[BAH2SS]

I dont track my car but i am using Redline 75w-90.

If its your track car then go with a 85W-140 this is based on what i've seen people use.

Redline & AMSOIL is what i recommend.

[2011SSCT]

Just had a complete rear end with 3:91's and a Truetrac installed in my car 2 weeks ago by a reputable shop near me. They put in Lucas 85W-140. They gave me two bottles to bring home and told me to swap out the fluid at 300 miles.


Again, it's a reputable shop and I'm just repeating what i was told. My car is not tracked, just a toy for around town.

[Black_SS10]

I run 75/90 in mine, conventional or synthetic it doesn't matter as the trutrac doesn't care. I use that weight because the bearings and the gearset used in a factory diff are speced for 75/90 gear oil. Drastically thicker oil may not lubricate properly. A strictly drag car may be different.

[Greenhornet2]

I'm not a fan of running a heavier weight oil in a posi. I had a shop rebuild the diff in my tahoe, just new bearings. When i got it back it groaned on sharp turns. I asked him what fluid he used and his response was that he used good stuff synthetic 85w-140 and he used a bottle of lsd additive, not sure which one. I don't tow, i changed it back to the called for weight 75w-90, it's back to quiet. My theory is it was too thick to lube the clutches properly. My theory could be wrong, but it was much quieter with the factory called for weight. If you track the car i can see going with 140. I don't believe you should try to cover up a gear whine with a heavier weight oil. It should be set up properly and it won't make much noise. If you are running an lpw cover, poly or solid diff and cradle bushings you will get more transfer of noise though. I don't get any whine from my stock gears with lpw cover, poly diff bushings, poly cradle bushings and posi mod. I do know that some gears can be noisier than others, but it sounds like you might not be up as good as it could be. I don't do my own diffs for this reason, they are a pain to do right especially setting pinion depth pressing bearings on and off.

[pnwdan]

3.91 conversion in mine and running the Royal Purple standard weight for our gears. Comes with friction modifiers already. Not a sound out of it.

[oldman14ss]

went for a country road run yesterday, put more miles on it. seems that the whine at excelleration is going away. coast still whine might be getting quieter. cant tell really. going to get about 300miles on it then on to the super slab for some sustained steady speeds of 75 and 80 to possibly work temper the gears more. OR should I wait till 500 miles at sustained speeds? gosh! the possibilities are endless! the oldman.........

[oldman14ss]

oh, almost forgot. did you guys experience any whine when you first broke it in and when did it stop? just paranoid. don't want to pull this thing again.

[frankwjr]

per Eaton's web site:


What kind of oil should I use for my Detroit Truetrac? Can I use synthetic? Do I need friction additive/modifier?

• A quality, petroleum-mineral based, GL5 rated 80w-90 gear oil should be used.
• Synthetic oils are not recommended for Truetrac applications. While some customers have found that synthetic oils works well in this application, Eaton has not confirmed under test conditions that all synthetic oils are compatible with Eaton Truetrac applications.
• Friction modifiers should not be used because they will decrease performance (reduce differential bias)

interesting about the friction modifier...


and with the LPW diff cover my rear is noisy as heck.


pic of my wear pattern when I installed the 3.91's





btw used anti seize as marker hence the silver color...

[xstuntman]

Just did the diff oil on my LS with one full quart of Lucas 75/90.

I was really surprised that the oil looked that black and had a lot of glitter after only 16k original miles. The drain plug was over tight from the factory and needed a nudge with the hammer too. After what I found I'm glad I changed it out. Is this an aluminum diff housing thing? I've done my share of 10 and 12 bolts and not seen this.

[Greenhornet2]

Yes it's an aluminum housing.

[Moto-Mojo]

That wear pattern pic looks like it’s dug into the ring gear on my phone. Doubt it is. It looks good with the possible exception the pinion could have been set ever so slightly deeper. Hell, I thought our diff’s had synthetic in them. I trust Amsoil.