Mix ATF conventional and synthetic?

[TBB2]

I'm driving a 2003 Chevy Express 3500 purchased used and want to change the ATF.
I have no information on what ATF is in the transmission now.
I am getting conflicting information on use of conventional vs synthetic ATF when draining and replacing the filter and fluid.
I am not getting any feedback on how to identify a fluid as conventional vs synthetic and assume it will require a serious chemical lab analysis.

First I am warned off of doing a flush because of the "grit"(?) in the gears is keeping it running smooth.
- But then others suggest to drain, replace, drive and repeat to get the most new fluid in the transmission.
-- That seems like a flush.
The Service Manual calls for DEXRON III which is conventional.
A local dealer uses DEXRON VI ATF; full synthetic for everything.
- Dealers in my opinion are not concerned with more than getting paid in the short term and stonewalling problems in the long term.
I have reports that synthetic can be used to replace conventional but not visa versa.
- But I was not intending to flush so whatever is in the trans now will mix with the new.
ACDelco does not recommend replacing conventional with synthetic.
- Some comments take ACDelco's stance as a cover-their-ass position so they can Pontius Pilate any complications.
Too many comments take whatever the local mechanic tells them to do.

Can anyone provide an in depth assessment of mixing ?

 

[dnewton3]

There is no real issue with mixing the base stocks.

Assuming the van has never had an ATF refresh, then the old Dex IIIh is in there currently.

You can use any fluid that would be backward compatible with that Dex IIIh. And that's a slew of products marketed as "Dex/Merc", or "D3/M", etc from a reputable source. The Dex IIIh fluid is no longer licensed, but many companies still make a reasonable effort to comply with the former fluid.

You could also use Dex VI. Or Maxlife syn ATF.

 

[kschachn]

I'm driving a 2003 Chevy Express 3500 purchased used and want to change the ATF.
I have no information on what ATF is in the transmission now.
I am getting conflicting information on use of conventional vs synthetic ATF when draining and replacing the filter and fluid.
I am not getting any feedback on how to identify a fluid as conventional vs synthetic and assume it will require a serious chemical lab analysis.

First I am warned off of doing a flush because of the "grit"(?) in the gears is keeping it running smooth.
- But then others suggest to drain, replace, drive and repeat to get the most new fluid in the transmission.
-- That seems like a flush.
The Service Manual calls for DEXRON III which is conventional.
A local dealer uses DEXRON VI ATF; full synthetic for everything.
- Dealers in my opinion are not concerned with more than getting paid in the short term and stonewalling problems in the long term.
I have reports that synthetic can be used to replace conventional but not visa versa.
- But I was not intending to flush so whatever is in the trans now will mix with the new.
ACDelco does not recommend replacing conventional with synthetic.
- Some comments take ACDelco's stance as a cover-their-ass position so they can Pontius Pilate any complications.
Too many comments take whatever the local mechanic tells them to do.

Can anyone provide an in depth assessment of mixing ?

Where is this statement by ACDelco?

And DEXRON-III does not specify a base stock composition. It’s a performance specification. But it’s also a superseded license specification as dnewton3 notes above. The official supercession is DEXRON-VI which is often a synthetic base. No covering of posteriors is needed, DEXRON-VI is a better license and a better fluid in every way.

I’d use Maxlife since I use it in every recommended application. But maybe that’s just me.

 

[JavierH19]

Maxlife and others should work fine but I've had the best experience using castrol transmax high mileage since it made first gear shift like brand new on two old 4 speeds. My sierra was a touch delayed but not anymore while the yukon was slightly quick but unnoticeable feel wise though you could hear the rpm drop quickly shifting into second when it was cold. when warmed up it was normal.

Looking up a pds after using it since it was recommended to me I saw that it has the highest KV100 being 8.0 so maybe that helps? or the additive pack is better for older more worn out transmissions as mine have over 600k combined with no rebuild so that could be it. I get mine from home depot in store but check if yours has it.

https://www.homedepot.com/p/CASTROL...atic-Transmission-Fluid-1-qt-151DD0/205159600

https://www.amazon.com/Castrol-Transmax-Mileage-Automatic-Transmission/dp/B00DILUC6U

 

[JeffKeryk]

I’d use Maxlife since I use it in every recommended application. But maybe that’s just me.

As a mere mortal, dare I ask if MaxLife is suitable for Toy WS or Honda DW-1?
Asking for a friend who services a lotta those cars. Thanks in advance...

 

[wlk]

I would run Valvoline MV Import synthetic or Valvoline EP atf which are both synthetic with a cst closer to Dex3. Maxlife atf is a little thinner, 1cst but is great fluid in older GMs as well. You can mix and/or run a synthetic in a older dex3 specd tranny no problems. I have had Maxlife in my 93 C1500 for >40k miles now with no issues. I was d&f with the MV Import syn but zon raised the cost to same as EP and $10/gal more than Maxlife so no real point. I had an Element that is was use the Import in as well but sold it. I will go with Maxlife again or EP at the next d&f.
Castrol also has very good fluids for the dex3s as well. Even Supertech Dex3 HM is a good option for it, just change is at 30k vs 50-60k intervals. id run whatever you can just keep it fresh and I'd also add Lubegard Red to it as well. It helped significantly with my harsh 1-2 gear shifts in several gms.

 

[kschachn]

As a mere mortal, dare I ask if MaxLife is suitable for Toy WS or Honda DW-1?
Asking for a friend who services a lotta those cars. Thanks in advance...

Well they say it is.

 

[TBB2]

dnewton,
Education on “base stocks” needed.
Assuming what is in the van now and mixing is the primary question. A
-----
kschachn,
The information from ACDelco comes from a CS conversation with a Parts Specialist.
On the ACDelco site and on retail sales sites DEXRON VI is ambiguous as to if it is synthetic or not. Some bottles say synthetic, on others there is no mention.
------
I am still hoping for some content on mixing.