I own a used 2003 Chevy 3500 Express VIN: last 8= 31110297. 74000 miles.
The manual calls for Dexron III ATF.

I know the previous owner did some service to the ATF but have no details. I want to change out the ATF fluid & filter just so I know what the service situation is.

Not knowing what ATF is in the transmission, I am perplexed over what now to use when dropping the pan and changing the filter. I have been warned off of doing an ATF flush and read that some fluid will remain in the transmission after the pan is dropped.

I am aware that Dexron III is superseded by Dexron VI.
But I have conflicting information on weather to use synthetic or non-synthetic (conventional) Dexron VI.

Can I use Dexron VI Full Synthetic to fill the transmission even if there is some Dexron III or other still in the transmission? And Visa versa, can I use Dexron VI conventional to fill the transmission even if there is some Dexron III or other still in the transmission?

 

You can swap out conventional fluid, oil or ATF with synthetic but you can not go backwards.
Information reported to be from a dealership.
: /
I do not harbor a lot of trust for dealerships.
And ...
that is not supported by the ATF fluid manufacturer.

 

There was a technical bulletin that said to use Dex VI for warranty repairs, and basically it will meet/exceed what you need for your transmission. It should blend just fine with any Dex III that is still in the cooler/torque converter. You can do a few drain and fills a few thousand miles apart and get close to a full fluid change.

As far as synthetic vs conventional I think it shouldn't matter as long as it's got the Dex VI approval. Others might know differently, but I wouldn't hesitate to mix as long as it's a quality fluid.

 

Burnt Clutch.

"it's got the Dex VI approval"

Is the "IT" in this case the ATF fluid?
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What about ACDelco instructions "do not mix" ?
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I'm surprised there have been no other responses.
Does everyone just go to a garage and cross their fingers?

 

Yup, that's what I was referring to.

I am admittedly very much not a transmission guru, so others might know better from first hand experience. But my thoughts are clean fluid is better than dirty fluid, regardless of if it's synthetic or conventional.

 

One of the first things I do with a new used vehicle is drop the pan, swap the filter and replace the fluid with high quality synthetic of the proper type. You never know what the previous owners put in there or if they did anything. I'll drive it for a bit then do another dump and fill to get some more of the old fluid out.

I've never worried about mixing conventional and synthetic for tranny or motor oil. Decades ago when synthetic motor oil became popular there were all sorts of warnings about mixing the two or swapping from one to the other. Nothing ever became of it and nobody even brings it up now. None of the shop owners or tranny guys I know worry about it either. If the type or rating of the fluid is updated then it will mix with the previous fluid.

I think some of the fear comes from manufacturers that won't OK anything without extensive specific testing that they aren't going to do. I just swapped in an Eaton Truetrac rear diff and they only recommended conventional gear oil. Even the printed paperwork that came with it stated that. When I looked into it it was just because they hadn't tested every synthetic oil with it. Now, the online version of those same instructions have been updated and say to use either conventional or synthetic.

I recently purchased one of these fluid extractors and have been impressed with it. I had to add an extra length of tubing to reach the bottom of our long filler tube but it was no big deal. It emptied the pan in a matter of minutes with zero mess when I did the second fluid dump. Well worth it IMHO.

 

It seems like people just don't like coming to a forum and discussing these vans. I'm on several other vehicle specific forums and they're all WAY more active than any Express/Savana forum I could find. This is the most active one I could find and I would consider it half dead. Even the forum for my Dodge minivan (2001-2007) is far more active even though it only covers a few production years. A thread like this would have dozens of responses by now.

 

This video link has an educated presentation:

Can Changing your Transmission Fluid Cause Damage?

 

BurntClutch,

Continued digging confirms your information.
I was really thrown by the lack of a response to the three emails sent to ACDelco after being told verbally by CS to not mix conventional and synthetic ATF.
At some point I had the impression DEXRON VI came in a conventional form but that seems to have come from poor labeling on the containers.
Another ATF producer was much more forthcoming with evidence that DEXRON VI ATF is the only approved fluid for warranty repairs to GM transmissions ... requiring DEXRON III ... . And including my vehicle in the list.
Up to now I had a high degree of confidence in ACDelco. I do not understand why they are tongue tied.