dexron III versus IV for modern PCM transmissions?

[DanielC]

Just because we use what is basically a car engine, does not mean we are using it in the same way a car uses an engine. the operating conditions, and stresses on the engine and transmission are different.

I would expect Mobile to suggest a higher priced synthetic ATF for you to use, because it is more profit for Mobile.

I am pretty sure if PCM thought there were advantages in using a synthetic oil, or ATF, they would recommend it.

[scottb7]

i just glanced at the 2012 pcm manual it it now shows that v drive tranny should have "mobiletrans shc50 synthetic lubricant"...i will one you that is smarter then me compare specs on this to dexron 6...

[MLA]

Dexron is Dexron. Since III was what was out when the unit was built and the manual was printed, the trans was built for the specs of the then available Dexron III. As the numbers go up, the new specs supersede the older specs by meeting and exceeding them. So, the current Dexron VI is a direct substitute, for the older (lower number) Dexron III. You can also go by the API (American Petroleum Institute) code. It works the same way. new API numbers supersede older API numbers, so they are safe to use in applications that originally called for the older API number.

[DanielC]

Do not trust a superseded API code with engine oils.
The Ford GT-40 engine has a flat tappet instead of a roller camshaft, and it needs the extra zinc and phosphorous used in the older oils. The zinc and phosphorous is hard on catalytic converters, so engine oils have been reducing it to low levels.
One oil with higher levels of zinc and phosphorous is Valvoline VR-1 Racing oil, Use 40W oil in your GT-40.

[jmo]

No synthetics in the tranny. The synthetic transmission fluid is actually too "slippery" and the transmission will slip.
Different story when it comes to motor oil. PCM does not allow for extended service if synthetic is used.

Agreed on PCM not wanting folks to extend the service life of fluids with synthetic, but...

Dex-6 is by definition for transmissions and is by definition synthetic and has better frictional characteristics than dex-3, look at these graphs from the link I orginally posted:

A --> Dex-3
B --> Dex-6

Clutch Durability


Viscosity over time


Resistance to foaming


Based on this data the argument to not use synthetic in the tranmission does not hold water IMHO...

At the end of the day I will go by what PCM suggests once they respond and most likely will pull the dex-6 out and put the dex/merc that castrol has in, but I just want to know why they haven't bothered to up rev to dex-6 since GM mandates it for all of their tranmissions that were supposed to get dex-3.

One other interesting tidbit I read somewhere is that dex-4 and ex-5 specs were never officially released by GM, they decided to jump to dex-6 since the driver behind the new spec was to support GM's new (at the time) "6" speed transmission. Atleast it made sense to someone :~)

Thanks for the discussion thread guys.

JMO

[j2nh]

Thanks jimo, my reference to synthetic transmission fluid came from TRBenj, I am going to send him a pm and ask for his reasons for saying no synthetics in the tranny.

I have always used Mobil 1 in the engine, but also kept to the 50 hour change rule. I also change the tranny fluid every 100 but have used non-synthetic. Never had any problems but these units are pretty bullet proof so I wouldn't expect any.

[Quinner]

My comment about the synthetic was specific to the direct drive tranny or any that originally used Dextron, not sure what fluids the modern Vee's recommend or use. Eric from Fantastic Finish Marine, a marine transmission re-builder, was who I have heard about not using synthetic for the reason J2 stated previously.

[scottb7]

I have seen similar dialogues on other vehicle forums. I am pleased to report that so far no one has lived long enough to have a tranny, engine, or differential failure from using a higher spec then originally called for with correct even near correct weight.

[jmo]

Hey guys,

Just wanted to give you the update from exchanging emails with PCM on this topic. As expected they only state the dex3 is what they recommend, and would not comment on why dex6 is or isn't okay to use. As I continued to ask for detail as to why - the rep just stopped responding. I guess customer service isn't that important to them given their customer is really Correct Craft...

The only thing new that I learned from them was that the PCM tranny is in fact their own design and not a GM product.

Now I have to decide if I want to run with dex6 or pull it and replace it with the dex/merc product most folks are using. I actually ran dex6 last year with no issues.

[DanielC]

Since it is a PCM designed transmission, they probably designed it for Dexron 3, the fluid commonly available at the the time it was designed, found it worked as designed, and recommended that fluid.
PCM could have spent a lot of extra time and money, trying different fluids, but why? That would have made the engines and transmissions more expensive. Dexron 3 works. No need to test the transmission with other fluids.

Just because a type of fluid has a higher number, or has a word in the name that has more to do with marketing than actual fact, does not mean it is better for your transmission. Just because your amplifier goes up to 11 instead of 10 does not make it louder. Purple dyes in your oils and fluids does not make then better. The word "synthetic" in the name does not make it better. It is mostly all marketing hype.

Use the fluids PCM recommends. It is really that simple.

[jmo]

Hi Daniel,

Yes I agree just using dex3 is the right way to go, my argument in regards to certifying dex6 is that GM is no longer certifying and licensing the dex3 standard anymore, so we have lost a safety check done by GM to insure the fluid sold in the auto parts store still complies with the spec PCM is after. So now we have to take the fluid manufactorer's word for it that it does in fact comply. The bottles of Dex/Merc in the auto parts store aren't labeled as dex3 compliant (that I saw anyways), the dex6 is explicitly labelled as fully compatible with dex3 transmissions - hence why I went down the dex6 path.

As I mentioned in an earlier post I found on Castrol website that their dex/merc product is engineered for the dex3 spec, so had I known that prior to my trip to the auto parts store I would have gone that route and been done with it. So for me the only question that remains is should I pull the brand new dex6 out, or just run it for 100 hrs and then switch back to dex3.

JMO

[DanielC]

I do not know if you should remove the Dex 6, and replace it, or not.
To be honest, I run Valvoline Dexron/Mercon formula in my boat. 1997 Ski Nautique, 2449 hours, and I use my boat for show skiing, so it has seen some time with more than the recommended two adult skiers max. I have also used it to tow docks, and jumps if needed for show setup.
The fluid come out of my boat looking brand new.

[jmo]

Yep - every time I change it I can't tell the difference between the old and the new. I think the application dex6 does so much better is in cars where the expected interval change is ~ 100k miles as that's where it's ability to retain its properties over time makes a difference.

That's amazing you have that many hours on your boat, I hope to follow in your footsteps.