I've read some search threads on this and now I'm lost. What ATF should I use in my '89 727 tranny? Thanks

 

Don't know for sure if you need the AFT+4 or not, but I prefer Amsoils Full synthetic in stock factory trannys. Running cooler is even more important in a stocker than in an aftermarket rebuilt! JMO. And the Amsoil will cover ATF+4, if needed.

RJ

 

Dexron/ Mercon III

 

Ditto on the Amsoil! good stuff, however, it's spendy!

 

And I love Hot Rod Tod's "Buy American" quote! I do whenever possible!!!!

 

It came from the factory with whatever the latest dexron was at the time.

 

Thank you sir, its something I feel very strongly about.

Now, I don't want to start an argument or a bash thread, but I would like you guys to think about something before recomending anyone to use synthetic oil in an automatic transmission. (SlushBox) Especially in a heavy vehicle with lots of torque, that will more than likely be used to tow something thats even heavier behind it, compounding the problem.
What causes most automatic transmissions to fail ?
1) Heat
2) Slippage
Are the two related ?
Yes, they are.
Synthetic oils are great at reducing friction, which is exactly why you don't want to use them in an automatic.
What usually fails in an automatic? The clutches and the friction plates between the clutch plates. An automatic depends on the friction between these two parts to transmit the engines power, to the driveshaft, and then to the wheels.
What happens when we remove the friction between the clutches and the plates ?
Slippage
What does slippage cause ?
Heat, not to mention, wear.
Do you see where I am going with this?
In a small, lightweight, low HP, low torque, passenger car, synthetic trans fluid may work just fine to help save a little fuel.
But none of us on this forum are using our auto trans for that. We are trying to move a minimum of 6,000 pound truck, with a mimimum 500 foot pounds of torque, stock. Not many of the trucks here are at the stock level, and probably haven't been since the first 5 miles on the trip home from the dealers lot.

So, lets say synthetic reduces friction by 1/2, ok now lets say you have 10 clutch discs in your forward clutch pack, basicly you have just reduced your clutch pack to 5 discs now, and reduced the clutches ability to transmit power 50%, by adding synthetic oil.
An automatic is a friction drive transmission, you want your trans to shift quick and firm to minimize heat and slippage, so that it will give you a long service life, thats why we put in shift kits, so they don't s l i d e into gear.

That is also why when you go to most performance tranny builders, they recomend you run Dexron III and a mixture of Lube Guard, (friction modifier) because the newer fluids are too slick, including Mopar ATF+4.

Don't get me wrong, I am not against synthetics, I put them in almost everything, just not automatic transmissions.

 

this makes good sense to me

 

Good logical post.
BUT - Sorry, not accurate!
There are many misconceptions about synthetics. Yours is one.
The only advantages to a synthetic lubricant are 1- thermal stability, and 2- most impurities have been removed... helping them last longer.
By "thermal stability" I mean synthetics just sit there and do what they are designed to do at much higher (and lower) temps! They are less likely to thin out, oxidize, break down or change chemically.
The only time they reduce friction over a mineral oil based lubricant is at very cold temps, because they are less likely to thicken up.

This explanation is greatly oversimplified but still valid.... and comes from 20 yrs working in the manufacturing end of the oil and lubrication industry.

RJ

 

I do agree with this part of you post. If someone wants to live with firmer shifts, less slippage is better. And if the earlier Dodges called for Mercon then it makes sense. It's not the synthetic base oils that product slippage, it's the extra friction modifier in ATF+4 that creates smoother shifts and some added slippage.I don't think some extra slippage is going to cost someone thousands of miles on there tranny clutch faces but it is worth thinking about.

I just believe that keeping fluid operating temps low is a more important consideration in a factory transmission. And a synthetic ATF will do that well.
This is especially important if owners wait 70,000 - 100,000 miles before changing filters and ATF!

All JMO!

RJ

 

The whole clutch thing makes no sense, there are small "slots" for the oil to escape from the clutches. Once the oil is gone, no slippage. The friction modifier makes the "cheap" oil the same as +4 for less money!!!

 

Did your 47re last 630,000 pulling a 4 car wedge?
Mine did.
Do you have a trans temp gague and a scan tool? I do and can back up my findings.
Load a friends truck on a trailer behind yours, start pulling a grade, check your trans temps and slippage with the synthetic, they both will be higher with synthetic vs dexron III.
Sorry, I have tried both fluids, I would rather change my good ol cheap dexron fluid every 15,000 rather than change my transmission once.
My tranny builder has been in business since 1958, he won't use synthetic in his builds and he won't warranty them if synthetic is in them.
I will stick with what has been proven to work, and last, over and over again.

 

I do have a gauge on my DTT. I have run both Amsoil's ATF and my companies, full synthetic Mercon III at the 1/4 drag strip! I found no lower times with the Amsoil and no higher fluid temps with the Mercon III.
Admittedly, this is not solid scientific data but it is not just theory either.
The difference in your CTD may be that the original design calls for Mercon and not ATF+4? Just a possibility.

RJ

 

I talked to a retired builder a few weeks ago-

He had NEVER seen an oil related failure. Friction discs, seals, and occasionally broken parts(exploded pistons etc). He was talking accross multi-makes.
Personally, I the bearings I've seen looked great in high-mileage transmissions.

M

 

Your saying he never saw a failure using something other than what is called for? And if that's the case it's because transmission fluids are basically the same base formula with different friction modifyer packages added.