What ATF should I use
 

‘65 Catalina 2+2 421 HO with Turbo 400 trans. Do not want to use a synthetic or synthetic blend. What conventional atf is right for this trans?

I always ran type "F" in mine. Went to a Direct Connection drag seminar many years ago and they said type "F" had a higher coefficient of friction. Worked in my GTO for 50 years. Interested to see what the experts here recommend.

I would run type F. I run type F in every TH350/400 I have.

Dex III or Type F

Last I heard, and this was decades ago, Type F made for firmer shifts, but was not as good as Dexron for lubrication, or for thermal stability.

Not a big fan of harsh shifts. They're fine with low-power, near-stock engines because maybe they chirp the tires 1-2, possibly even 2-3. But with REAL power, you just blow traction on the shifts, and shock-load the drivetrain--transmission guts, U-joints, differential, axle shafts.

Type F does nothing I want.

NO idea why you're resistant to a synthetic or semi-synthetic fluid.

Temperature is by far more important then fluid type as long as the fluid is not crap.

I would install a trans cooler that is big enough to do away with the fluid running into the factory cooler that is in the radiator.

I have run the Ford fluid for 60k miles in an everyday driver and GM in another .
The only thing I would say is to not mix them if your changing and can’t drain the 4 QTs out of the converter .

Since the TH400 is a GM Product i would use what GM recommends, Dexron II, III or IV.

Type F is for racers, they break their stuff all the time and don't mind fixing or renovating them.

JMHO

Dude that built my T400 recs ATI Super F and it has been flawless. I ran Dexron for years with great success in my stockers

I run dex III in my stock 3 speeds. I run synthetic in my built overdrives as per recommended by hughes.

I don't have a problem with any of it

Dex3 is ideal, supertech make a good quality ATF for best price, Ive been running that in a few vehicles for 20-30 years, high mile daily driver and pontiac t350/400s, have it in a TCI street fighter t400 behind a 500+hp 467 that runs low 11's, works/shifts excellent on the track or street.

Used type F in a higher mile stock t400 in a 78 TA for 50k+ miles behind a mild 455 back in the late 80's through 90's, beat the crap out of that car as a teenager/20's kid, it had a soft 3rd gear shift at WOT and the type F improved that a lot... never had any heat related issues driving in 90+f summer temps with no aux cooler. The stock rad cooler will keep temps within reason as long as the engine doesnt overheat, mine ran at 180-190 in all temps.
If youre in a cooler climate, or any climate, you dont want to bypass the rad cooler, removes most the heat first then route to a aux cooler for the rest... for street cars, aux coolers dont cool at a stop or low speeds. Thats how every aux cooler company recommends to run them. pure drag race is different.

Another very good ATF is Valvoline Maxlife, meets specs for may different brands including dex3, its synthetic but doesnt cause any problems like leaks etc, it has seal conditioners to help leaks and will handle a lot more heat/stress than non synthetic fluid. I did a full exchange on my Toyota 4 runner that calls for non synthetic toyota WS ATF, most 4runner/tacoma owners swap it out with excellent results, mines been doing great for 5+ years now, shifts much smoother and runs cooler with maxlife.

Supertech TGH tractor fluid in my ATs and PS boxes. Have not run the red stuff for the last 15 or more years and I will never go back.

Valvoline Dex/merc been using it for 40+ years

FWIW I’ve had synthetic Dex VI In the th400 in my Firebird that was used when I got it. Been in there since like 2017. No problems.

GM says it’s backwards compatible and that’s been true in my experience.

I like Type F. But in this case the proper level of red fluid is what matters most.

Dex/Merc or type F if it's never been rebuilt, Dex 3 would work as well. The additive package in Dex 4 and up has a lot of detergents,etc and can finish off weak seals. Type F has pretty much no additives.

Above I recommended Type F. I have found the older large/heavy cars seem to shift very lazy with Dex after they get some miles on them. Type F will firm/quicken the shift up and not harm anything. It's harder on the trans to shift slow/soft.

50+ year transmission rebuilder and I use Dex 3/Mercon

Just keep in mind that in old cars with an auto trans the smoother the shifts are the faster the clutch packs and bands will ware and that slipping that makes the smooth shifts also makes for higher fluid temps.

Modern auto vehicles shift smooth because the during a shift point the computer drops the timing way back for a few milliseconds.

I was a Type F guy for a long time. used some Dex 3 because I got it free. It ran the same, same ETs and 60 foots and all. But its a race trans.
Doubt it matters at all for a stock deal.
Now, doing a little trans work right now. Drained out my ATI Type F, looked and smelled new after 2 seasons. That stuff is expensive. I ran it though a couple a those thick shop paper towels in a funnel.
Will top it off with new synthetic type F

https://www.amazon.com/dp/B0FJQDBWNN...n_title_2&th=1
synthetic

I use 4 quarts dex 3 for color and additives, the rest Hy-Gard John Deere transmission fluid because its cheap. The turbo racer guys swear by it on YB. There is a standard viscosity and low viscosity Hy-Gard. The standard vis will lower your converter stall speed about 200-300 RPM. I use the standard viscosity and it does tighten the converter a bit at cruise.