What to use in a 1968 4-spd transmission

[bronze turbine]

I have a service manual for a 1968 Chrysler and it says to use the Dexron ATF in the 3-spd manual transmission. I have a 4-spd manual in my '68 300 (not original to the car). I want to top it off, but I don't know what's already in it. If it's filled with ATF would topping it off with 85w90 gear lube cause any problems? In other words, would the the ATF and the gear lube mix without any issues? Or, should I top it off with ATF? If I were to drain the whole thing, what should I fill it with?

 

[CBODY67]

Gear lube and ATF look, feel, and smell totally different. Gear lube would smell like what's in the rear axle, for example. Just sampling what's in there should indicate what it is.

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Just looked in the FSM for 1968 Plymouths I free-downloaded at www.mymopar.com . Seems you can use either SAE 140 Cold Climate gear oil, SAE 80 or 90 gear oil (normal rear axle lube), or Dexron-family ATF (replaces the previous "Type A, Suffix A atf). Most of the current Dexron III ATF oils are now named "Multi-Make", but reading the oil container, it usually mentions "Dexron" as the main one it replaces. Current Dexron VI is a full-syn fluid, so the old Dex III fluid might be a more economical fill.

Obviously, using atf would result in less drag on the engine than the thicker 80W-90 lube oil would. Might even increase fuel economy on the highway!

CBODY67

 

[Big_John]

I'd drain it and fill with the fluid of your choice.

 

[fury fan]

FWIW, I'm seeing mixed reports on Dex 6, some are saying full-synthetic, some say it's a blend.
Based on 30 secs of review, it seems it's based on the brand/label.

The biggest concern will be whether the fluid has enough friction for the synchronizers and stop rings to work properly.
Modern fluids and automatic transmissions are designed to work as a system, so the latest oil might not be a good choice.
Backward-compatibility is likely OK for the gears and bearings, as modern ATs use those.
We are likely using some bronze bushings, modern ATs will be using high-tech plastic or a metal bushing coated with Teflon. New oil is likely OK with our stuff.
Seal compatibility is another potential issue. If a trans has had seals replaced, might be OK. 50-year-old seals might start leaking.

But most likely, if it shifts OK, it'll be OK.