Adding a higher type of ATF to older ATF

[StaleFuel]

Years ago I filled up the 1991 Chevrolet S10 manual transmission with AAMCO Synthetic Blend Mercon V ATF (exceeds Dexron III/Mercon). At least that is what I think I used. I believe the truck calls for Dexron III.

I can't get that fluid anymore.

Can I add Valvoline Dexron-VI Full Synthetic ATF, VV370 ?

Thanks.

 

[simple_gifts]

If you are tied to using a synthetic fluid, perhaps you should consider a dedicated synthetic MTF rather than an ATF.

 

[StaleFuel]

No, not really tied to synthetic. I just have some Valvoline Dexron-VI Full Synthetic ATF, VV370 sitting around, that I would like to use on it.

 

[kschachn]

Are you sure the owner's manual calls for Dex III? Can you check?

If it does you can try the Dex VI and see what happens. It is backwards compatible but I've never thought of it that way IRT a manual transmission. It works fine in my old BMW, but that's an automatic. My ECHO is a manual but it calls for GL-4 or GL-5 which is what I use.

 

[StaleFuel]

It is definitely automatic transmission fluid. The only part I can't remember if it is Dexron II or Dexron III. I am pretty sure it is the III. I don't have the manual with me right now.
I just don't know about mixing different grades of fluid.

 

[901Memphis]

Yes DexronVI is fully compatible and superior to DexronIII and DexronII. It has the same frictional properties as DexronIIIH but better base oils so superior longevity.

 

[StaleFuel]

So, I don't have to drain the old fluid, I can mix the two, just to make sure I understand you correctly?

 

[901Memphis]

Originally Posted By: StaleFuel
So, I don't have to drain the old fluid, I can mix the two, just to make sure I understand you correctly?


Yes you can mix them although the old fluid is probably in pretty bad shape and getting it all out wouldn't hurt if it's an easy job. If you're just topping off a leak I understand not wanting to drain all the old fluid, especially since most don't have normal drain plugs.

 

[69GTX]

Not all DEX VI's are full compatible to manual trans missions formerly requiring DEX III.

The Tremec T56 is specced for Dex III. GM warns not to use Dex VI in 2007 and prior units. Tremec doesn't even mention Dex VI as a choice.

 

[4WD]

Mobil 1 synthetic ATF can replace the older Dex ATF ...

 

[mattwithcats]

GM replaced it with -

Genuine GM Fluid 88861800 Manual Transmission Fluid

This also works,
https://www.walmart.com/ip/Valvoline-Dexron-Mercon-Automatic-Transmission-Fluid/15125788

Careful, the red jug is Dexron VI...
And the quart bottles can be either color, read the label carefully...

 

[901Memphis]

ATFs are a compromise when used in manual transmission but they work and are cheap so it's what's used.

 

[Gasbuggy]

I thought it was common knowledge, no Dex 6 in a manual trans. Buy some plain Dex III.

 

[slacktide_bitog]

Yes, Dexron VI is backwards-compatible to all GM transmissions

 

[901Memphis]

Originally Posted By: Gasbuggy
I thought it was common knowledge, no Dex 6 in a manual trans. Buy some plain Dex III.


DexronIII no longer has any licensed products so you have to gamble on Dex/Merc products if that's the route you want to go.

I still don't understand why people claim DexronVI doesn't work in manual transmissions calling for DexronIII since the coefficient of friction is about the same and the DexronVI is more shear stable.

The DexronVI does have a much lower starting viscosity, but it would stay high when the mineral DexronIII has sheared to nothing in a couple years.

For performance applications it's always best to try to find a proper manual transmission fluid even if they specify an ATF. Sometimes you have to play with viscosity ranges to get the desired shift feel. No doubt they specify ATF for the MTF because it's easier than creating the proper fluid.

 

[Gasbuggy]

Tremec put out a PR years ago to prevent Dex 6 going into their transmissions.