Ok, watched the video. Kind of skewed in his testing as I don't care how the oil pours when cold, I want to know it won't turn to water when it is hot inside the transmission and clings to the gears - kinda useless test. And gear oil whipping around in the transmission causes loss of power? Hmmm, must have been why I got beat by that Rambler with the 401CI and blower on top.
How did we ever get by in the "old days" by simply going to the parts store and picking up a bottle of 80W-90 gear lube and never questioned its molecular make-up or friction modifiers? The same stuff you put in the trans, you used in the rear end and then if you had posi, used an additive.
From my understandings, "our old" gear oil was a GL3.
So what in the hell is GL4, let alone GL5? I did a little research, so this is my opinion. The GL5 is definitely NOT what you want in your Muncie or any other "old car" manual transmission, period. It is for newer vehicles designed for such lubricants. The use of a GL-5 lubricant in a synchromesh transmission with brass synchro's will "shorten the synchronizer life by one half".
Next is the GL4. GL4 fluids may or may not be right for the older transmissions. It all depends on what additives have been introduced into the formula - with each manufacturer/supplier having their own unique formula.
The key is that you want to steer away for the letters "EP" or "XEP." EP= Extreme Pressure additives prevent seizure conditions caused by direct metal-to-metal contact between the parts under high loads. These additives can be: Chlorinated paraffins; Sulphurized fats; Esters; Zinc dialkyldithiophosphate (ZDDP); and Molybdenum disulfide.
Why steer away? Some which contain phosphorous/sulfurous compounds (but not all depending on the EP compound and how much is used) are corrosive to yellow metals such as the copper and/or brass used in bushings and synchronizers on the older manual transmissions.
However, most all GL4 gear oils have some form of EP compounds, so you can't really get by without them. You just want to know what the EP used is and less of it is better.
In reading online, it seems the GL4 gear oils are going the way of the GL3 that we used to use - obsoleted or more difficult to get as the GL5 and even GL6 becomes the gear oil of choice for those cars still even using a manual gear box anymore.
There are a number of choices to include synthetics that are claimed to be used with the older (Muncie) transmissions and ARE COMPATIBLE with the brass synchro rings and soft metal bushings. Looking around for recommendations I found these"
produces an SAE 85W90 GL-4 gear oil (SL24229) which is perfectly suited to all Muncie, Super T-10, and most T-10 4-speeds. Though originally most Muncie, Super T-10, and T-10 units came from the factory with 75W90 conventional gear lube, it is becoming very hard to find 75W90 conventional in the modern era – 80W90 and 85W90 are more commonly found and both can be safely used as a 75W90 replacement. Sta-Lube SL24229 will protect all the components in these classic 4-speeds and this includes their synchronizers. And it works very well in original Muncie, Super T-10, or T-10 units where the seals have never been replaced. And this is especially important since synthetic gear oil tends to leak out of older transmissions at a much quicker rate than conventional gear oil. https://www.crcindustries.com/produc...z-SL24229.html
offers a GL4 Synthetic Manual Transmission & Transaxle Gear Lube that includes use with brass synchro's and for older transmissions (Muncie, T-10, Top loader, etc.) It is formulated with seal conditioners to reduce leaks. https://www.amsoil.com/shop/by-produ...r-lube-75w-90/
offers their MT-90 75W90 GL-4 Gear Oil synthetic gear oil for older transmissions. Recommended for GL-1, GL-3 and GL-4 applications and safe for brass synchros, as it lacks the reactive sulfurs found in most GL-5 oils that cause damage. https://www.redlineoil.com/mt-90-75w90-gl-4-gear-oil
also offers the same mix in a slightly lighter oil viscosity with MT-85 75W85 GL-4 Gear Oil. https://www.redlineoil.com/mt-85-75w85-gl-4-gear-oil
also offers this mix, MTL® 75W80 GL-4 Gear Oil. https://www.redlineoil.com/mtl-75w80-gl-4-gear-oil
There may be other options and brands as I did not try to rifle through all the blogs to see who is using what, but these above seemed to pop up more often in the older car categories. This simply gives you an idea of what to look for and options that can be used in our older manual transmissions. I miss the good old days when I could wake up and walk outside naked, reach down on the ground to that leaking spot under my Muncie, and say to the dog, "There's nothing like the smell of transmission oil in the morning."