Guys- Exposing my newbieness.... how do you change the fluid on the trans? I see the drain bolt on the side, but wheres the fill spot?

That is the fill hole.

There is no easy way. You have to use a suction device of some sort and a tube to suck it out the best you can through that fill hole. Or you remove it from the car so you can dump it out through the filler hole. If I were you, I would not mess with the fluid unless you know it is really old. The aftermarket Muncie cases come with a drain plug. Autogear in Syracuse will drill and tap a hole for a drain plug.

I think I need to... no idea when it was changed last, and based on both your description of the clumsiness of the process, and the lack of effort the prior owner made on her as I'm finding out, I better get into this.... Thanks Bob~
Is 80w90 CL4 correct oil? Seems to be what the internet is telling me

I agree with Bob. It's gear oil and you never change it unless you had excessive debris in there. "Can't find it grind it".

I have a very old suction device made for removing and adding oil into transmissions, rear dif etc.

I will post a pic. You can get a bottle of gear oil with a hose attachment. That will work.

To remove old oil:
- open/remove upper oil plug on transmission
- note the level of oil, put your finger in the upper hole
- open/remove lower oil plug on transmission, have bucket under plug to catch oil
- when all oil removed, put bottom plug back in
To add new oil:
- put hose on top of bottle
- place hose in top plug hole, tilt bottle bottom end up, squeeze oil into tranny
- your full when the oil starts to come back out. let it drain so the most you have is oil to the fill hole (top plug)

Gus, Muncie's don't have a drain plug

Certainly not that I can find for sure!

I think the service manual for both the manual transmissions and rear end state no periodic change of oil is required. Clearly if you do a major repair it happens by default.

True, there is no service schedule for the gearbox like the engine-one year or so many miles deal, but that gearbox has most likely had debris ground into the oil for the last 40+ years, can't do the gears, bearings, syncros etc. any good. I say get a fluid extractor and change the oil. Try to find some 80/90 GL-4 spec, the same lube that was put in the diff housing and transmissions back when these cars were made. GL-4 has been replace by GL-5 but the Muncie wants GL-4 spec 80/90 oil. The GL-5 has sulfur additive and can harm the Munci's syncros. Phillips 66, Valvoline, Chevron etc, and others still supply GL-4 for older transmission brands. I put GL-4 in my M20 along with a bottle of GM synchromesh additive when I changed mine. You should have seen the crap that came out with the oil. I avoid synthetics.
Just my opinion.

Be aware that they started putting limited slip additive in rear end oil (valvolene). It eats up brass synchros. My 3-4 gear synchro is shot and it may be from me using the valvolene 80w90 in my Muncie. Make sure you get manual transmission oil.

Yikes. Maybe I'll just crack it open, plug it, and check out the plug, assuming its magnetic, maybe I'll just look for any particles and close it up with the new magnetic plug...

Oh crap. Going out to look what I just put in my Saginaw.

Luck would have it, I put in the wrong oil. Drained/siphoned it out and put in new. Do you think I should run it a few miles and then drain this and put in new to make sure it all gets flushed out?

What oil did you put in?

I had a T10 back then and it came with 2 plugs.

GL-5 Gear Oil, 80-90W.

What brand? I just looked up ac Delco gl-5 and it specifically says it does not have limited slip additives. If the stuff you used doesn’t you should be find

It was Carquest brand. There isn't crap on the label about not using it in manual transmissions with syncros. This statement from the website (not on the bottle) got me concerned.

CARQUEST Premium SAE 80W-90 Gear Oil contains sulfur and phosphorous additive metals, but no zinc,

With the valvolene 80w90, I bought a quart and noticed it didn’t say it was for manual transmissions. I had an older bottle of it and looked at the label and it did list manual transmissions. All the text was the same except for that. I poked around on the internet and found discussions on valvolene putting in limited slip additives and that it eats the brass synchros in transmissions. The new bottles don’t warn you not to use it in transmissions. Just doesn’t list transmissions as the recommended use

I dumped the oil out yesterday. With the Saginaw, it is much easier to get the oil out, so I'm sure I got 90+% out. I'm thinking about running it a few miles and then dump the oil again, then refill. I just rebuilt the transmission a few months ago (all new syncros) so I would rather not have to rebuild it again.

I don't think it's the limited slip additive that is the problem but the greater amount of extreme pressure additives in the GL-5 gear lube over what is in the the GL-4 gear lube. The extreme pressure additives are a concentration of active and inactive sulfurs which can react to copper and copper alloys, brass and bronze. There are GL-5 gear lubes with no limited slip additive as well as GL-5 lubes that do have limited slip additives. They both contain the sulfur which is bad for "yellow" gear boxes. "Yellow" gear boxes have copper and copper alloys in the syncros, other gearboxes have no copper, bronze or brass in the syncros and can use different lube than the yellow boxes. Limited slip additive is supposed to be a mixture of friction modifiers including Liquid Wax Ester. There are limited slip differentials that use copper alloys in the clutches, putting high sulfur lube in those diffs wouldn't help the clutch pacs at all.

According to the Society of Automobile Engineers Paper 2007-1-1988 - fully formulated API GL-5 is unable to fulfill the lubrication requirements of limited slip differentials. Some makers of GL-5 add limited slip additive to their lube others don't. I bought three quarts of Pennzoil lube off the shelf at the local garage, two had the limited slip additive one didn't

In my opinion, one should check the API rating, if it isn't GL-4 don't put in your muncie, if it's GL-5 whether it has limited slip additive or not, don't put in your Muncie.

"limited slip differentials" are features of the rear diff. Gearboxes have GEAR OIL in them. Once put in, they rarely get changed. Only situation I can think of is if the brass syncros are trashed then you would want to swap out old gear for new stuff.

Now, there is an additive for limited slip and Safetrak.Put it in the rear diff if necessary, just don't put it in the transmission.

I hope your rebuilt saganow is numbers matching. If it was made from trees it would be a balsa-wood tranny.

For my 6 banger Sprint, it is more than adequate.

Good info, Bluebird. Thanks!

X2 blue. Thanks for the info.

Likewise to everyone, you guys probably save me another transmission rebuild.

Been years since looked at my 67, but pretty sure has a drain and fill plug.
And about new Valvoline ok or not. Maybe check their website.

I found some gear oil in my garage that had the mil spec not the API spec on the bottle. The 1968 Pontiac service manual says, to use SAE-80 or SAE-90 multi purpose gear lubricant MIL-L-2105B in the manual transmission. Mil spec MIL-L-2105B Is the same spec as GL-4 lubricant, MIL-L-2105C,2105D,2105E is the same spec as GL-5. Thought this may help if you find some gear oil with the MIL spec only.

Also read a transaxle paper by a Corvair guy, he states GL-4 has only 1/2 the sulfur/phosphorous additive that the GL-5 has. That results in less bonding, and less extreme pressure protection, so the additive peels off parts easier and as a result takes less brass off the surface. In used oil analysis they found 2-4 times as much copper in transmissions using GL-5 as they did in those using GL-4.

The manual doesn't say anything about using synchromesh additive in the transmission, I never put any type of additive in motor oil, but I did put a bottle of GM synchromesh additive in my Muncie when I change the lubricant. Seemed to shift better, but that could have been the oil change, or maybe me self justifying spending the money on the additive.

After reading all this great info--- I forgot to ask the simplest: How many quarts does the M20 take?

I found this on Amazon.... seems like the right stuff? $17 a quart

That looks like the right stuff. Here is what I used.

2-1/2 pints.

Thx Guys