BJ67

Does anyone know if the M7 trans fluid is 100 % synthetic from the factory fill ? and same question on the 75/90 gear oil in the rear end, is it 100 % synthetic oil (non ELSD)??

BJ67

Anyone??? I am thinking about doing a complete drain of both and go 100 % synthetic unless the factory fill is all ready synthetic.

X25

The factory fill diff fluid's bottle does not state that it's synthetic anywhere on it. That said, I don't know if there is any approved product besides OEM part # for diff fluid. For transmission, you could choose any brand. I went with Amsoil's signature ATF

Kracka

I went with Redline D4 ATF in my M7 transmission and it feels quite a bit better than it did with the OEM fluid. For the rear differential, I stayed with OEM rather than having to mess around with aftermarket synthetic fluid + limited slip additives.

Here's a write-up I did on the procedure: https://www.corvetteforum.com/forums...d-service.html

And a few other related threads I've started to reference on the matter:
https://www.corvetteforum.com/forums...tr-6070-a.html
https://www.corvetteforum.com/forums...7-shifter.html

TEXHAWK0

I would just stick with the GM approved fluids.
I have friends that switched to synthetic trans. fluid back when I had a C5 and it actually shifted worse... They changed back to the GM fluid and it smoothed out...
Without knowing if compatible with ALL internals, I would not go to an aftermarket fluid.

FKING1

Redline D4ATF in trans & Reline 75/90 synthetic in Diff.
Ran this combo in my C5 for over 10 years with no probs.

Kracka

The compatibility you're looking for is Dexron III ATF, per TREMEC.

X25

The problem is rather with the diff fluid. GM's LS gear oil is not certified by most manufacturers, though same fluid is recommended for C5/C6 as well as many other cars, and they do work with regular diff fluids from other manufacturers with no issues reported.

Kracka

Exactly why I stuck with OEM for the rear diff. The biggest question was, do I have to add LSD additive, and if so, how much? In that regard, I figured GM already did that for me and packaged it into a bottle!

X25

Makes sense. On my last C6 Corvette, I went with Amsoil's 75W-110, which already has the additive, too, and is also more stable at high temps than a regular 75W-90, while being fully compatible with 75W-90 (75W-110 was released later to address concerns about 75W-90 being too wide of a spec, and is more focused on high temp stability and performance). For my C7 Z51, I will use the OEM fluid for the time being, but will replace it often. I might switch to another fluid later on, once I get to know the car well, and make enough lap times with the OEM fluid. Once I have the data, I can then use another fluid, and see if I screwed myself up : )

Greg00Coupe

I threw in Amsoil in the rear end. Problem is how do I know it is any better or worse? Assuming I have no mechanical issues? I'm thinking of changing the trans out........ I can tell better shifting or not on the tranny. Friend is an amsoil dealer and the high mile cars he has make me a believer.