Air in the Gearbox oil

[Rata Road]

I drained the gearbox oil in my 39 Dodge yesterday after running it on blocks for awhile and was surprised to find it full of air, very foamy. Even after a long period the oil still has a thick layer of froth on top? Looks very clean, just frothy.

Have you ever seen this?

[EmTee]

What color?  Milky?  Could there be moisture/condensation build-up from not getting warm enough to drive out the moisture?

[Rata Road]

No its not milky in any way the oil is clean.

[plymouthcranbrook]

What kind of oil is in it?

[Rata Road]

I'm not sure as I've only had it for a few weeks.

I checked the container today that the oil drained into and its still frothy on top 24 hrs later?

I might take a pic of the drained oil tomorrow .

I checked the fresh oil today after approx 45 min drive and its not frothy. However the syncro on 2nd gear is still non existent. Seems to go in nice every now and then but mostly crunches going both up and down!

 

 

[plymouthcranbrook]

When I get a moment to go to my garage I will take a photo of what I put in my 52 Plymouth and post it. I had crunchy second gear shifts unless I really shifted slow.  This oil cured it completely. As to being “frothy” since I have never really drained mine hot from the trans I have never seen or heard of that. My only idea would be that the oil that was in it was way to thin. Since some manual transmissions use thin oil from the factory perhaps someone just put in the wrong stuff.

[plymouthcranbrook]

This is what I used and let’s see if I can make a photo magically appear:

 

 

 

[Rata Road]

Thanks for that input I will see if I can find that or a similar rating.

I just checked the drained oil and it is still frothy (45 hrs) but has settled down to about 1/4" froth. It was about an 1" deep or more when drained warm.

I stuck a screw driver into the oil and as you can see in the pic the oil is clean but also very thick when cold.

Its worth trying that synthetic. 

[EmTee]

x2 on Redline MTL!  I have that in the T-10 in my '64 GP and it shifts like butter. 

[EmTee]

1 hour ago, Rata Road said:

I just checked the drained oil and it is still frothy (45 hrs)...

Seems like it must be contaminated with something - but what, I don't know...

[Rata Road]

I cant seem to find it in NZ so far and I see Amazon are out of stock, I'll keep searching...........

[Stude Light]

When I drain the SAE 90 or SAE 250 gear oils that I run in my prewar cars they will be really frothy if I drain them right after running. It is pretty normal as the gears churn up the oil. It takes a day or so for the oil to lose all the froth. Afterwards the oil looks just like new. I use GL1 or GL4 oils depending on the application. I’ve recently done this on a fresh rebuild with new oil so it isn’t a contamination issue.

[Rata Road]

Great to hear, I wasn't going to throw the oil out as it still looks clean. 

 

[Rata Road]

Ive found a similar Redline oil.

This one states 75w80 GL-4., Is it the exact same as what your thinking?

 

 

[Bloo]

Redline's synchromesh transmission friendly oils, in order from lightest to heaviest:

 

1) MTL >> 2) MT-85  >> 3) MT-90 (or 75W90*NS*) >> 4) 75W140*NS*

 

The first 3 are for transmissions only, the *NS* oils are intended for transaxles that contain both a synchronized transmission and some hypoid rearend gears running in the same oil supply. They are fine in transmissions. Since MT-90 exists at the same viscosity as 75W90*NS*, I probably would use MT-90 over 75W90*NS* in a transmission even though either one would be fine. I am using 75W140*NS*, the heaviest one, in the 1936 Pontiac transmission and it works great. I've used MTL (the lightest one) and MT-90 in several other cars. My experience with these has been all good. 

 

Note: Redline 75W90 and 75W140 without the *NS* are not suitable for synchromesh transmissions.

 

Edited May 28, 2023 by Bloo (see edit history)

[Rata Road]

I would prefer to try the exact same oil as the guys above as it has helped with the poor syncro going into 2nd gear both up and down. Unfortunately their pic of the MTL shows no numbers.

Thats the only reason I will be changing out the brand new oil I've just put in the gearbox, only to see if I can avoid a rebuild by using this oil.

The gearbox has been partly rebuilt by the previous owner but they must have skipped the syncro. A shame as the gearbox doesn't whine in any gear. I'm waiting to view the receipt to see exactly what they replaced.

 

 

[Rata Road]

Here is a list of GL-4's I can get locally, Is any of these a exact match?

 

Red Line MT-90 75W90 GL-4 Gear Oil *not for use in differentials with hypoid gears*

Red Line MT-85 75W85 GL-4 Gear Oil *not for use in differentials with hypoid gears*

 

Red Line MTL 75W80 GL-4 Gear Oil *not for use in differentials with hypoid gears*

[Bloo]

I'd probably go for the mt-90 out of that lot in an old car because it is the thickest of the three, and they are all pretty thin. I think any of them would be fine, and @plymouthcranbrook already mentioned that his 52 is working great with MTL.

 

GL-4 doesn't mean much when we are talking about transmissions. It and GL-5 are ratings for hypoid gear service, GL-5 being the severe service one. 30 or 40 years ago there was a rule of thumb that people liked to throw around that GL-4 belongs in a transmission and GL-5 is for an axle. It was a happy accident that it sort of worked out then, but has nothing to do with what the ratings mean. In those days GL-5 always had more sulfur in it and was more expensive. Now there are synthetics, and special purpose oils for synchromesh, some synthetic some not. Meanwhile the hypoid rear axle oils have got so good that sometimes gear synchronizers wont even work in them.

 

There are many synchromesh oils these days. It doesn't have to be Redline. Redline is what I like and is readily available here, but I have no idea what other options might be available to you in NZ.

 

[plymouthcranbrook]

Once again as an old shade tree mechanic I would expect that the any of the listed oils would work well. After all how many miles per year are you driving?  It probably would take forever to wear out the parts of your trans in the amount you probably drive.  After all any modern synthetic for manual transmissions is going to be infinitely better than the original oil. 

[Rata Road]

I'm not just looking for a oil that will just work, the oil I have in it will do that I am solely looking for an oil (if one exists) that will help take the crunch out of my 2nd gear selections that the existing oil wont.

If I cant find one I have to make a choice, live with it or rebuild the gearbox.

[Bloo]

If anything will help this will. It's made for synchromesh. If the synchronizers are completely shot, or if the clutch is dragging, nothing will help.

 

Many modern gear oils are too slippery for synchronizers to work properly. Synchronizers are brakes. It is a fight between the synchronizer trying to scrape the oil off so it can stop the gear, and the oil trying to stay on the braking surface and keep the gear spinning. One of them is going to win, it's just a question of which one.

[John E. Guitar]

Have you talked to one of the oil companies? They have very knowledgable people on their industrial lubricants help lines. I've always had good advice from them for my vintage & veteran cars. 

[Rata Road]

Thanks Guys

[Stude Light]

I spent several years as a manual transmission development engineer at GM and @Bloo is spot on. 
 

If you look at your synchronizer assembly closely you will most likely see a brass, bronze or aluminum inverted cone with grooves in it that rides over a polished steel cone. The gear oil creates a boundary layer between the two parts to prevent wear but when you shift, you are squeezing that oil out from between the tops of the grooves (ridges) and the steel cone.

 

Once the synchronizers wear, the tops of those ridges flatten out and get wider. The valleys of the grooves get smaller and its harder to squeeze out the oil to get the synchronizer ring to brake and lock on to the gear cone to allow the two parts to speed match. 
 

For worn out synchros, I recommend using a mineral based oil without the EP friction modifiers required for a hypoid. That said, I suggest a 90 wt GL-1 gear oil. Generally the synthetic oils are more “slippery” (they have better lubrication properties) which isn’t what you want for worn synchronizers.

Scott

[Rata Road]

Thanks Scott that's very helpful.