GL-5 vs GL-4 Gear Oil

I've read that the reason why GL-5 gear oil should not be used is that it is not friendly to yellow metals which may be present in various parts in old tractors. The truth seems to be more complicated. In fact, GL-5 is compatible with yellow metals because the sulfur is "buffered" and so does not corrode yellow metal. However, because of the greater quaintly of EP additive (the sulfur) present in GL-5 if can cause a problem of wiping away a thin layer of brass from some parts (synchronizers) when under load, whereas GL-4 which doesn't have as much EP doesn't do this (however it does allow more wear to happen to other parts). So, since a JD-40 and pretty much all tractors lack synchronizers in their transmissions, it would seem GL-5 is just fine. Agreement on this? Anything overlooked. Thanks in advance.

To be truthfull, I'm not sure anyone understands the problem & certainly not me. Everyone say's "yellow metal" but you have more brass in them now than ever before & the higher numbers are the most modern oils. My guess is there isn't a problem with yellow brass at all. It will be the red brass (bronze bushings) that causes issues. Even then it's the water that alway's gets inside corroding the copper content and not the yellow metal. I'm sure someone has some actual data explaining this and I would sure like to see it if it exists.

It was originally posted on the N board, but may be that THIS will help. Good info all the way round.

Noah W

M-Man has it pretty well covered.
GL-4 if maked as marine gear lube. It"s formulated to resist corrosion and still provide lubrication even when a seal has failed and the gear box is water contaminated.
There is GL-5 and GL-5 yellow metal friendly . Yellow metal contains copper so that would be brass and bronze.
Back in the bad old days . Operators of trucks etc would reduce gear box failures . By adding a spoon full of sulphur powder to the GL-1 oil of that era. The sulphur would leave a hard surface finish in the steel gears.
Does sulphur corrode copper? Have a look at old copper battery cables around a lead acid battery.
Many automotive manufactures spec motor oil in manual transmissions. Odd however many people will dump auto transmission oil in an engine and fuel tank as a wonder additive. Yet they are terrified to put a super duper corrosion inhibiting lube with excellent pressure point wear protection called motor oil in a transmission?
Few people also understand that 90wt gear lube has the same viscosity as 40 to 50wt motor oil.

Motor oil in transmissions? The Japanese motorcycle riders have mostly been doing that for many years. Hondas coming over here in early 1960s or so specified 10-40 motor oil and it was both engine and transmission lube in the unitary-engine and tranny sharing common case and lubricant- engines designs. GM in 1980s Pontiac Sunbird specified 10-30/10-40 motor oil in the manual transmissions- quick change oil place put in gear lube instead when I went there with coupon for "full service" and a month later when snow came I was stuck in gear until warmed up. Couple weeks of get off work in morning and 1st gear only- I tended to park in gear- drive around parking lot at idle 10 minutes before could shift meant read the manual- gear lube recommended only in south, OK for Texas but not Wisconsin in December. Some other cars have the motor oil recommendations for manual transmissions, some say auto tranny fluid in manuals also. 15-40 Diesel oil good for trans and implement/machinery gear box"s that say "mineral oil" or motor oil- the diesel oils have Zinc additives the newer gasoline car oils don"t have since zinc fouls cats like lead. 40 weight non detergent is good for summer use in some old design garden equipment- is Ariens tiller tine drive suggestion for worm drive with brass gears. Some uses for motor oil when manuals are read, some are older equipment specs with current upgrades noted. For some real fun with older limited slip differentials- try to find the recommended sperm whale oil. Alternative for some is a bean oil. Castor bean oil for a few gear units- some old English products like a David Brown garden gear box on 2 wheeled tractor, a tiller with 2 stroke Villiers engine that also suggest using castor oil in engine. RN

The problem with sulfur is under certain conditions when mixed with water it can create sulfuric acid. The acid is what attacks the bushing.

If the oil has MT1 somewhere on the label then it is buffered for use in manual transmission. As Owen Aaland mentioned the real problem for tractors is case condensation that creates sulfuric acid.

It's mainly a problem for tractors that have to sit out in the weather IMO. If you keep your 40 under some kind of roof and use it once in a while it will be fine.