[lakeroadster]

Just as the title states: What Ya Using For Lube on a M-21 Muncie? My '65 C-10 has red trannny fluid in it from the previous owner. I need to change the lip seal in the tail housing, so nows the time to choose the right lube.

[cobalt327]

Here is a post covering it: muncie fluid change with no drain hole

Good luck.

[lakeroadster]

Thanks for the link.

I am wondering how big a deal it is in regards to flushing the trans to get the ATF fluid out before filling with Sta-Lube GL4 from Napa?

[cobalt327]

Originally Posted by lakeroadster

Thanks for the link.

I am wondering how big a deal it is in regards to flushing the trans to get the ATF fluid out before filling with Sta-Lube GL4 from Napa?

If the trans in in the car, do two fills.

First fill run for a short distance just to thoroughly mix the new lube w/the remaining ATF (if that's what it was and not some sort of synthetic gear lube), then dump. Second fill keep for the duration.

The smell will tell you if it was ATF if there's any question, obviously if it was piss thin you know it's not right because the correct gear lube is very viscous compared to any ATF.

[66GMC]

I can't say that I've ever heard a recommendation to use ATF in an M21, but I do recall Ford's recommendation of using it in a NP 203 transfer case on a Ford 4x4 in order to improve cold-weather shifting.

Those T-Cases also had a little oil pump in them as well, and I suspect that they mave have been another reason behind using a lower-viscosity lubricant than the 80W90 gear oil, which is thicker than molasses at -30°.

[cobalt327]

FWIW there are several common manual transmissions that use ATF type fluid (like the T-5 for one), but like you said- sure not a Muncie 4-speed.

The OP could have a synthetic manual trans lube like Castrol BOT 402- it's red and has a GL 4 rating. But it's also thicker than ATF: SAE 75W-85. But at least it would be the right lube.

I'd do the smell test before assuming it's ATF, though. The two (ATF and gear lube) smell markedly different. Hopefully it's been filled w/gear lube and it just looks like ATF.

[lakeroadster]

Thanks guys, I 'll switch it over to Sta-Lube GL4. I found this article while searching the internet.

ATF in a muncie? - THE H.A.M.B.

I had a 1985 S-10 with a 5-speed manual, it used ATF in the tranny. But on my ole Muncie that thicker GL4 will help to keep my truck from "marking it's territory. Well the thicker lube will help, but the new lipseals and shift arm seals will no doubt help more.

Thanks again,

John

[gino21]

What ever you run steer clear of synthetic gear oil it doesn't allow the synchros to do their job especially at higher rpm cause it's. too slippery.

[AutoGear]

Gino is correct.
Also, The fill/drain hole is located incorrectly. I wish I still had the service bulletin but, the actual measurement is to fill it till you can insert the tip of your finger and bend it at the first knuckle and touch the fluid. They designed it around a 1qt capacity. Over filling it is actually as damaging as under filling it.

You can run a Muncie on 1qt with no averse effects. It will run cooler and shift faster. The P/N for the Sta-Lube is SL24229 @ Napa. I have a lot of customers who also use Brad Penn and the only synthetic I can recommend (mostly for people who can't find the Sta-Lube) is Royal Purple's MAX GEAR in a 75w-90 or there abouts. Only the MAX GEAR though. You do not need an additive package with any of these oils.