Synthetic GL5 gear oil - it is bad for brass?

People have posted concern that synthetic gear oil deteriorates brass. The story is that GL5 forms a protective black coating on the brass. But when surface load is applied to the brass that black coating comes off taking some brass with it. On the other hand - running your gearbox without the GL5 can result in more wear on the steel gears so some people run it in their box anyway - preferring the extra protection for the expensive gears.

I've been using a portion of GL5 oil mixed in with regular gear oil in all my boxes for over 20 years. One of my boxes (hot rod 1949 Willys 4WD Truck) endures a lot of abuse so it gets rebuild whenever its time for a complete tear down. It lasted till the rebuilt engine wore out and when I looked at the brass syncros - one was in perfect clean shape and the other had black stain on it. The stained one was completely worn out. This could be from abuse or from the synthetic GL5 but I have doubts its the GL5 because one of the syncros shows no wear.

So have any others seen brass deterioration that is defititely attributed to using GL5 or is this hyperbole talk?

Photo below shows the black stained worn syncro on the right.



The image below right shows wear on the back of the syncro (the ID is also worn)



Its possible that running only a 50/50 mix of GL5 synthetic with regular GL4 gear oil avoids brass deterioration and that I may have simply overheated and overworked the tranny from my many heavy load firewood trailer hauls.

I watched a YouTube video of a guy who rebuilds gearboxes for a living and so has seen the effects of different oils. He has only ever seen some darkening of the bushes using synthetic or mineral GL5 oil and nothing else. I use synthetic gear oil in all my bikes and they have been fine. I use a 75/90 one that says it is GL4 & GL5 compliant.

There should be no brass in a Norton gearbox, the bushes are bronze.

Brass is similar to bronze, another alloy containing copper that uses tin in place of zinc

GL5 alone does not tell you it attacks yellow metals, it's the sulphur based additives and not all GL5 oil contain the sulphur additives. If it does the bottle should have a warning.

Has anyone asked the gear oil manufacturers?

Onder said:

Has anyone asked the gear oil manufacturers?

Click to expand...

I asked Bel Ray once, and got this reply telling me which of their gear oils are GL4/5. Note they state that the GL4 types are suitable for soft yellow metals:


Good Morning Nigel,

Thank you for contacting us. The Bel-Ray V-Twin Sport Transmission Fluid is appropriate for use in transmissions that have soft yellow metals such as brass, bronze, or copper.

The Bel-Ray V-Twin Big Twin Transmission Oil and both of our Bel-Ray Gear Saver Hypoid Gear Oils (80W-90, 85W-140) are GL-5 gear oils.

The Bel-Ray V-Twin Sport Transmission Fluid, Gear Saver Transmission Oils (75W, 80W), and Thumper Gear Saver Transmission Oil 80W-85 are not GL-5 gear oils, and are suitable for use in motorcycle transmissions that have soft yellow metals.

Best Regards,

Cody Bartkowiak
Powersports & Automotive Technical Support

kommando - brass or bronze - they are primarily copper and both would be effected. The negative posts mention more copper showing up in the oil with CL5 oils.

Nortoniggy - I'm not sure the jury is in or has agreed yet.

I use a mix of regular GL4 gear oil and amsoil GL5 gear lube or severe gear lube. I also use a little redline shockproof as an additive. None of them have sulphur warnings so my example may not show sulphur related problems. I'm not sure if some of these oils bother with the sulphur warning or not (I have doubts).

Here's an interesting article by a Triumph rider who immersed bronze bushings in GL 5 oil for two years and took measurements (no problems with yellow metal soaking in a bottle):


And below is the doomsday report:

I had a VW T5, always main dealer serviced, from new and at around eight years old it started to get difficult to smoothly change gears. The diagnosis was the wrong oil had been used from the day it was filled as it had the tell tale gold glisten to the oil.
I wasn't impressed, even less so when I had to cough up a grand for a, used, replacement.

So GL-5 preserves the gear teeth whilst eating away at the bushes.GL-4 is benign bush wise but less protective of the gear faces.
With any old box you are now left doing the calculation of which bits die first.
My answer?.....ALL of them ! :-O

My initial take on all this a while back was “yawn... more nerdy blah, blah” but...

The bottom line for me though is that both gearbox manufacturers and oil manufacturers agree that GL5 is not suitable for situations where there is soft yellow metal.

Also, a good modern GL4, whilst perhaps not being as ultra protective, or long lasting as GL5 (modern applications keep the transmission oil in for tens of thousands of miles, and we just don’t do that), GL4 is still infinitely superior to oils of old.

My conclusion is that a top quality GL4, changed fairly frequently is more than adequate for our application, therefore we can save the worry energy for something else...

jseng1 said:

kommando - brass or bronze - they are primarily copper and both would be effected. The negative posts mention more copper showing up in the oil with CL5 oils.

Click to expand...

The distinction between brass and bronze matters when you buy bushes or valve guides from some cowboy who makes them out of brass because it’s easy to machine.

Back to the beginning: which gear is the syncro on? One would suspect the lower gears work it harder. The wear looks like mechanical wear not so much yellow metal being eaten by evil compounds.

Back in the 1960s I was told not to use Castrol EP90 in Lambretta gearboxes because of the bronze bushes but to use Castrol ST90. As an apprentice I did as I was told and never questioned it.

Reading this thread, I thought it might be a good time to change my gearbox oil. It must have done at least 15000 km :
Castrol MTX synthetic 75W-140 GL 5.
Drained the oil, no sign of bronze particles..

Onder - It does look like mechanical wear on the syrcros. Many years of gear grinding does that. The primo syncro makes me suspect the warnings about GL5 oil and bronze.

Triton thrasher and kommando - When I refered to "brass" I was talking about syncromesh gears which can be made of either brass or bronze.

Fast Eddie - GL5 is getting a bad rap for yellow metal oxidation but my own experience with partial mixes of GL5 doesn't show a problem. A couple others mentioned in this thread see no problem with it either. Talk is one thing but where has an actual emperical test or comparison been made?

Considering their are many synthetic 75W90 gearbox oils that are GL 4 spec, don't see an issue. Stocked in more retailers than you can find EP90. I would also suspect, like FE, that any modern GL 4 is far better formulated these days

jseng1 said:

Onder - It does look like mechanical wear on the syrcros. Many years of gear grinding does that. The primo syncro makes me suspect the warnings about GL5 oil and bronze.

Triton thrasher and kommando - When I refered to "brass" I was talking about syncromesh gears which can be made of either brass or bronze.

Fast Eddie - GL5 is getting a bad rap for yellow metal oxidation but my own experience with partial mixes of GL5 doesn't show a problem. A couple others mentioned in this thread see no problem with it either. Talk is one thing but where has an actual emperical test or comparison been made?

Click to expand...

I too would like to see any data we can find on GL5 Jim.

Currently, it seems we don’t have firm data it is a problem. Then again, we don’t have firm data that it offers any actual meaningful benefit in a Norton box.

So, in that way, they’re neutral.

But, then when we look further afield, the makers of Norton boxes (TTI at least) and the makers of GL5 oil, both say not to use it.

Therefore, untill such times as the gearbox and oil manufacturers change their stance, or are proved wrong, I know what I’ll be using.

...cue mad rush to drain the fleets gearboxes...

Hi Jim
Those synchro's off the same gear set? Or is one carrying a lot more load or spinning higher etc?
I work with/for a Caterpillar engineer and we had a discussion yesterday regarding the bronzes and different gear oils, and he said it has no effect on the various bushes in Caterpillar transmissions etc, we were looking at what oil to use in an older surface grinder we had rebuilt and trying to deicide on the correct oils.
I have been working with Cat gear since 1978 and back then you could use just basic oils in nearly every compartment, ie transmission and hydraulics used the same oil, I didn't go along with that and put hydraulic oil in the hydraulics and transmission oils in the transmissions, end result mechanically was about the same but we made significant savings in oil cost, but at some confusion with service guys.
Cat gear runs for somewhere 10000 hours to 18000 hours with great reliability, the advantage they have is they mostly run 24 hours per day except for down times servicing so there is very little heating and cooling, condensation etc, whereas our old bikes have lots of heating cycles and opportunity for condensation.
Conclusion from me is I don't think it makes a big difference, you really need to look at the loads on those synchro's from a mechanical/engineering point of view and calculate which one has the biggest load, when engaging, so to see if it a oil issue or service issue plus a heap of other issues, basically you need real data.
I don't think it makes a lot of difference in our old bikes.
Burgs