Ok, so winter is coming up, and I'm a compulsive nut and like to change all my fluids every year before winter. Now I've read that GM has switched from Dextron III to Dextron VI and that any vehicle that uses Dextron III should switch to Dextron VI.

So, question is: Should I switch to Dextron VI or to Mobil 1 Synthetic ATF. I've read that the Mobil fluid can be used in all GM vehicles 2005 and older. I've used Mobil engine oil since I bought the car, and will swear by it, but I don't know much about trans fluid and if synthetic make a difference.

Also Dextron VI and Mobil Synthetic ATF are about the same price per liter. I'm guessing that Dextron VI is probably a synthetic version of Dextron III?

Thanks,
Ashton

When I rebuilt my T56 transmission on the T/A last winter, I had a long discussion with the guy I bought all my upgrade parts from. He has rebuilt 100'* of transmissions, and races on the weekends. He told me synthetic is less prone to breaking down under prolonged high heat conditions, but that regular transmission has better lubrication properties. Unless you are running your car at the track on a regular basis, he recommends using regular transmission fluid, not synthetic. I would also say if you are changing your transmission fluid every year, I would not spend the extra bucks for synthetic.

I'm sorry, I don't know if Dexron VI is synthetic. Your local parts counter guy should be able to tell you that.

Here'* some info on Dexron IV LUBE REPORT: GM Rolls Out Dexron-VI

Basically the Dexron IV is for 2006+ cars with 6 speed Hydra Matic transmissions. Personally....I would stick with Dexron III - it'* less expensive and exceeds recommendations for your year of car.

My opinion, go with Dextron III and Lucas. Since you change it so regularly if there was an advantage, like 7500 miles between engine oil changes, you would not get it for the added expense.
Mike

I've rebuilt a couple of our transmissions as I'm learning a lot recently and wanted to save some labor expenses.

In doing so I talked with many transmission guys, specifically on our transmissions and lubrication/fluid choices. Not one recommended anything beyond a standard Dex/Merc III fluid. Especially when changing each year.

In a race application there is a desire to thicken the fluid with something like a Lucas product. The reason for this is the differential is spinning fast and most of the failure in the diff is lubrication related. A thicker/clinging fluid keeps it lubricated better.

In my race built transmission, I'm running Autozone'* brand Coastal, because it'* thicker than most other fluids. Then I am also using an additive.

In closing, the lower cost cheap fluid should work as well as the expensive fluids if not a bit better.

Interesting read. Looks like I'm going to stick with Dextron III.

Thanks for the advise

p.* The reason I change it so often is that I had it rebuilt about 2 years ago. When it was apart the shop that did the rebuild said that it failed because of overheating and the fluid being "cooked". So I slapped on a B&M tranny cooler and temp guage, and make sure that fluid is clean.

When I first got my car I had the trans rebuilt as it was totally shot, 40k later I changed the fluid and filter, at that point I switched to Dex VI, been about 10k miles since then everything still fells just as it did 50k earlier. According to the bottle it is a perfectly acceptable replacement to Dex III.

It is an acceptable substitute. The Dex III is up to the task, and way less expensive.