Stock T-98A transmission gear lube?

[David Zelinski]

My stock T-98A came back from the transmission builder today. Good ole' southern man, in his late 70's. Considered one of the best in the business around here. He told me to use straight 40 or 50 weight motor oil in the B/W T-98A instead of 80-90 Gear lube and also to stay away from anything synthetic or synthetic blend.

Anyone ever heard of this before?

[Don Cavey]

Not really. I thought that synthetic was the better choice?

[Barrman]

The important point is using a lube that is "yellow metal" safe. Straight 40 or 50 weight probably won't be safe for the brass, bronze and other parts inside the transmission. My other thought was that the thinner oil might not handle the load our heavy trucks put on them for very long.

Synthetic should be good to go as long as it is meant for transmissions. The only trouble is that synthetic is made up of smaller molecules than dino oil. So, if you have a leak now from the rear seal, it will be a bigger leak with the synthetic. However with your new everything, all should be good with synthetic.

[bullmoose]

The important point is using a lube that is "yellow metal" safe. Straight 40 or 50 weight probably won't be safe for the brass, bronze and other parts inside the transmission.

I believe the straight engine oil or GL-1 would be great for these parts (brass), I believe what the gentleman is referring to is GL-5 gear lube as it contains sulfer that is bad for brass parts such as syncro rings and bushings.

MHO, Lee

[Duane]

Red line makes a GL-4 75-90w oil that is good for syncro mesh transmissions, it is marketed as a low sulfur for use in brass syncros, it is called MT-90, im sure there are many others.

Duane

[jeepdan]

What about the transfer case that gets push to the limits on todays highways? Will any of the modern gear lubes help with that?

[bullmoose]

IMHO, the synthetic would be the better choice, even more so in the t/c, I run it in both, no issues.

Lee

[boonie]

I have a local heavy equipment trans & diff shop in my town,his advice was if it sets alot with synthetic oil any thing inside not submerged in oil is subject to rusting ,I never knew that! & to use mobile 5000 motor oil, it will not turn to foam like others.His testing of oils is with a bowl full & a hand crank egg beater, I also added a bottle of lucas oil stabilizer . boonie

[bullmoose]

I have a local heavy equipment trans & diff shop in my town,his advice was if it sets alot with synthetic oil any thing inside not submerged in oil is subject to rusting ,I never knew that! & to use mobile 5000 motor oil, it will not turn to foam like others.His testing of oils is with a bowl full & a hand crank egg beater, I also added a bottle of lucas oil stabilizer . boonie

All good advise, I was not aware of the rusting issues, Thank you!

Lee

[randyscycle]

To add to all the conflicting info, I'd add this:

The transmission was designed to use 90W gear oil when it was new. I assume that in rebuilding your transmission, new bearings were used and any parts replaced were old stock otherwise. The only thing that would be of modern manufacture most likely would be the seals and gaskets. Bearings haven't changed much and the seals wouldn't care much, so the gears, synchronizer rings and shift forks are the critical lubrication parts that need what they were designed for.

Given that, I'd go back to 90W gear oil and let it ride. Besides, I'd imagine you aren't going to be driving this truck 50,000 miles a year. Synthetic oil probably won't give you a benefit you'll even notice, and the thinner 40 or 50W oil may show some lubricity problems once its hot.

Just another 2 cents.