Which ATF for 1950's Dynaflow?

The latest is DEXRON 6 or VI. I have been using DEXRON III. Which DEXRON would be best for 1950's Dynaflows?

Joe, BCA 33493

Some bottles of the stuff say it is backward compatible except for some exotic manual transmissions...then again those making that determination probably never heard of a dynaflow. My transmission builder who does my dynaflows recently did a mult-speed auto tranny from a Jaguar that call for their own special fluid. It performed well on Dexron since in his words "the materials inside were no different than any other transmission including dynaflows". He says to use any Dexron, but never Type-A sold at some convenience stores.

Thanks for your response old-tank. I heard somewhere that the newer DEXRON had an additive that had different friction properties and would not be as good as DEXRON I, II or III in old Dynaflows. I still can get some DEXRON III, so I will stick with that.

Joe BCA 33493

Dextron VI I looked at was full synthetic. I found some dexron/Mercon at autozone. Another post on this site said it was what to use.

mines not up and running yet, so I can't say for sure...

Synthetic is probably ok, but tough on old seals. I Like the dextron red stuff, mineral oil based. It helps you identify the rear seal is leaking. Buy a bunch when its on sale, as you will likely leak. It's also dynamite for convertible tops (again, to see the leaks) and excellent for de-sludging old engines. You also run it in your diesel truck now and then to clean injectors and carbon. One reason dynaflows rarely take a poop is they get a continuous oil change out the rear seal- always look for the positive!

The GM/ACDelco-supplied Dexron Vi is "Semi-synthetic", although some manufacturers have some "full synthetic" Dexron VI just as these same companies also built full synthetic Dexron III. According to GM, Dexron VI is backward-compatible to the first GM automatic transmissions, BUT Dexron III should NOT be used in anything which specs Dexron VI (i.e., the new GM 6-speed automatics, or newer).

Dexron III is still around, just now termed "Multi-Make". IF you read the back of the bottle, it'll list the main application to be GM Dexron III use, plus some Ford uses, too. Another "multi-make" fluid lists an import use as its main application . . . so reading the back of the bottle is highly important.

About the time GM came out with the Dex VI, Chrysler also came out with a new trans fluid spec, which turns out to also be semi-synthetic in nature. An article on it mentioned that there was a particular seal in prior automatics which would soften if a full-synthetic atf was used, so they chose the semi-syn fluid spec instead.

The old Type A Suffix A automatic trans fluid was what all automatics were generally spec'd for prior to the use of Dexron (GM) and Type F (Ford) fluids in about 1968 model year. As time progressed, the Dexron family expanded to include new spec upgrades to the final DexronIII spec, ALL being backward compatible to the first GM automatics. As a result, the difference in performance of DexIII in comparison to Type A would be huge . . . in automatic transmissions. Power steering units might be another situation, though! Since 1962, GM's had a unique Power Steering Fluid in their chemical catalog, just as Chrysler had a unique power steering fluid, too.

Enjoy!

NTX5467

Nice little disclosure read NTX5467, By the way, U stated: " As a result, the difference in performance of DexIII in comparison to Type A would be huge . . . in automatic transmissions "

Could you elaborate on this a little further, specifically in what performance parameters would that be a benefit , etc ?

We have heard that the original Type A had a titrated amount of whale oil in it and Type III may not be so friendly to older type seals and clutch type surface materials if my memory serves me right.

To me, the "huge difference" would be in the much better additive package and (very possibly) a higher quality oil base stock, when compared to the original Type A fluid. This would very possibly mean better heat tolerance over extended periods of time, better detergents for cleaner valve body passageways and valve/modulation mechanisms. Still being mineral oil-based, I have heard of no issues of Dexron III causing any seal issues in earlier transmissions . . . but then again, only the "high mileage" variations (as Valvoline MaxLife Dexron III atf having any "seal conditioners" already in it) might help compensate for that.

The fluid viscosities are apparently pretty close, but I suspect the later Dexron III might flow a little better than the earlier fluid did. This could mean a more responsive "engage" function and possibly a quicker fill of the torque converter after sitting overnight (IF that might be an issue). In cases where shaft/seal wear might be an issue, the "better flowing" fluid might get out a little easier, though. Might also wick through cork-style pan gaskets, too, but the fix for that (where ever such gaskets are used) would be to put a thin coat of black, high temp silicone on all sides of the gasket (upper, lower, inside, and outside) prior to installation (letting it cure overnight) to encapsulate the gasket in a thin layer of silicone . . . stops the wicking action and also makes for easier clean-off when the pan needs to be removed the next time (with a little care, might even make the gasket "re-useable", possibly).

Clutch/band material compatibility would not, I suspect, be an issue.

With regards to Dexron III use as a power steering fluid . . . from my experiences, when Dexron III has been put into a later model (in this case, my '67 Chrysler before I bought it) and there is enough to turn the psf red, I noticed a "weeping" of fluid through the power steering pressure line. When I purged the system, as best I could by draining the reservoir a time or two, refilling with genuine Chrysler psf, the weeping stopped. If genuine Chrysler psf is not available, the similar GM product can be used.

In many "service manuals" used by service station mechanics, in the 1960s-70s, it was stated that you could use atf to top-off a power steering reservoir, which indicated that psf was ok to use in power steering systems. Compound that with the fact that Ford used atf (up to Mercon IV) in their power steering systems, but GM and Chrysler were not like that. In any event, atf could be used to top-off the power steering reservoir ONCE, if it was not very low, but once the main component of the power steering fluid system became "red" atf, then fluid seeps could occur through the hoses. Change the system back to the approved fluid and they'd stop.

Clearly, ALL rubber is spec'd for the fluid(s) it will come in contact with, whether O-rings or hose, by observation. GM makes a specific O-ring for their THM automatic dipstick tube seals. Put a normal O-ring in that place and it'll leak in about two weeks. Use the OEM GM item and "no leaks".

Still, though, GM states the latest Dexron VI fluid is "backward compatible to 1949", so it then becomes a matter of whether you want the semi-synthetic Dexron VI or the non-synthetic Dexron III fluid in your transmission. All GM now sells is Dexron VI, with the aftermarket being the only sellers of Dexron III and Ford Type F atf. Type F has a little more aggressive initial engage fluid coefficient, which is why the hot rod guys put it in their GM automatics, back then. But using Dexron III in a Ford automatic not spec'd for it (OEM) might cause a little initial-apply slippage of the clutches. As I recall, Type F is not that friendly to lock-up torque converters?

Just some thoughts . . .

NTX5467

Thank you.