Forgive me if this subject has been addressed already, but I did several searches and found nothing. I wanted to create this thread to inform other T-56 owners because this issue is not very widely publicized.

My car is mostly stock save for some bolt ons, cam, full UMI front suspension, and some other little things. It is used to get me to college and take trips from home to college on occasion (250 miles).

I took my car to the dealer approximately 5 months ago for a LS7 clutch, Flywheel, Tick master, and some new fluid. A couple weeks ago the car started to pop out of 3rd gear. My trans after 167k is all stock save for the MGW and Bronze isolator cup. I just thought that my 3rd gear synchronizer and/or blocker rings were done in and that time had taken its toll on them.

I was wrong. Through some research and talking with an engineer at Tremec, I came to the conclusion that the Dex 6 used by the dealer is the wrong fluid.

Dex 6 was introduced in 2006, when the GM licence for Dex 3 expired. Dex 6 is a fully synthetic ATF and replaces Dex 3 in all applications EXCEPT MANUAL TRANSMISSIONS AND POWER STEERING SYSTEMS (stated directly on the back of the AC-Delco Bottle [part # 88861037])



Attached to this post is the Dex 6 service bulletin from GM and a document from Tremec on the T-56. Notice on the first document on the first page that the same caveat on the back of the bottle is found in the bulletin.

Now here is the tricky part. I went to an ABC parts house to find the above mentioned part number and their computer cross-referenced a new part #. Now the new bottle does not have the same statement on the back. Interesting...

The fluid that goes into the T-56 is called either manual transmission fluid (also for transfer cases on 4x4 applications) or syncromesh. Now my understanding of this is that the syncromesh is used for warmer climates and in applications that will see more track time.

Here is the issue with MOST synthetic ATF's in your stock or stock replacement T-56. Synthetic fluids have superior heat dissipation, viscosity indexes, and film strength. The stock T-56 materials for syncros and blocker rings is a carbon-paper type of material similar to sandpaper but obviously much more durable. Now doing a simple mental or even physical experiment with sandpaper, mineral based oil, and a high quality synthetic oil you can see which oil allows more friction on the surface.

Basically the superior film strength, overtime causes a type of glaze or protective coat on the friction surfaces. This causes the friction material to not quite speed up the next gear adequately causing premature failure and aggravation.

If you have a T-56 that has been gone through you most likely will not have these issues. Bronze/Carbon fiber Syncros and blocker rings don't have such limitations regarding fluid type.

As for my situation. I am currently working through it. The dealer has been very cooperative and accommodating because they realize the wrong fluid was used. I wanted to make this tread for everyone that is still running a stock T-56. Maybe it will save them from some frustration. The orange tag on the trans that says "Dex III Only" is there for a reason.

 

This seems to suggest Dex 6 is OEM fill from 2006 onwards in 6 speeds.

 

That's correct. The C6's I believe got it first.

 

They made some internal changes.

 

The A/C Delco part number is 88861800 that is the correct G.M. oil for your t56, you do not want to put Dex 6 in any manual transmission. There are several TSB's (technical service bulletins) that state this is the correct G.M. Oil.

There are several bulletins that say Dex 6 is correct but they have all been superseded with bulletins that say DO NOT use the Dex 6 in manual transmissions. And the preferred G.M. Oil for manual transmissions is the A/C Delco part # 88861800.