Draining the gearbox of my 1969 Triumph Bonneville 650 at the end of the 2017 season I discovered gold! But actually, lots of bronze in the oil.
An inspection of the gearbox revealed that one of the bronze layshaft thrust washers had spun on its steel locating pin and was gouged out by the pin.
The sight of all that bronze in the oil was shocking. At first I couldn't understand how such a small amount of bronze could produce so much jetsam! What about the GL5 gear oil I'd used? I'd always heeded warnings about yellow metals and GL5 gear oil and only used it that season because it was all I could find.
"Yellow metal" refers to bronze, copper, and brass. On older motorcycles, the yellow metals bronze and copper were commonly used in gearboxes and transmissions for bushes and thrust washers.
Bronze and copper are good at their jobs, but they are known to react with, that is to say they are dissolved by, certain chemicals, including sulphur.
It turns out that sulphur is one of the main additives that oil companies use in their gear oils. Sulphur is added because it forms a protective coating and it increase the oil's slipperiness.
So, sulphur forms a protective coating, and then eats away the bronze. Terrific!
What I'd always heard was that gear oils conforming to the GL1 or GL4 standards were Ok for the old bikes, but that GL5 and GL4/GL5 products were not.
A look around the Web confirmed that GL5 products generally have a significantly higher sulphur content than those conforming to the GL4 spec. The added slipperiness of the GL5 products is important to modern transmissions and gearboxes.
One might conclude, or at least concede the possibility, that higher concentrations of sulphur in the GL5 additive package could potentially be detrimental to the yellow metal parts of vintage motorcycle gearboxes.
When I posted my GL5 concerns to the Britbike.com forum it was met with skepticism by a significant number of listees, all of whom have a great deal more experience and knowledge than myself. Still, a search of the Britbike and TriumphRat forums did turn up some credible concerns about GL5 oils.
I also found a 2014 post in another forum in which a supposed Lucas "product manager" advised that Lucas gear oil is NOT yellow metal friendly and should be avoided when they are present. At the same time, the poster alluded to a new product designed to be yellow metal safe. ("Riverbound" on riverdavesplace.com, 'Don't use our products with yellow metal')
Many opinions. What are the facts?
Having a couple of identical used bronze bushes laying around, as well as partial containers of two different products (GL5 and GL4/GL5), I decided to experiment by immersing the bushes in the GL5 oils for an extended period of time and compare before and after outside diameters.
The before measurements.
Pouring the oils over the bushes produced one surprising observation: the Valvoline was uniformly the color of dark honey, while the Lucas product was much lighter in color and began as a clear stream but turned into a plume of super-fine suspended particles near the end of the container. Additives? Sulfur? Shake well before using?
Research shows that damage to yellow metal from sulphur and other additives occurs more quickly at higher temperatures: 40C and up. Standard tests of the corrosive properties of lubricants are made at temperatures of 100C, but for only 24 hours ("Lubricant Testing 101: Copper Corrosion" Nye Lubricants on YouTube).
Doubtful that the temperature in the Bonnie Castle ever exceeded much more than 30C. On the other hand, this trial ran for something like 700 times longer than the standard 24-hour, high-temperature test.
In April, 2020, after they'd been immersed in their respective gear oils for nearly two years, I fished the bushes out of their jars and wiped them off. They were both so shiny! Especially the Lucas!
The moment of truth! I put the micrometer to them, and... ? Exactly the same. No discernible difference from the beginning measurements.
As noted, the trial lacked the customery testing component of high temperature. Nevertheless, over a significantly long duration of time, neither the Valvoline or the Lucas GL-5 gear oils had any effect on the bushes other than keeping oxidation from forming on them.
Some oil companies acknowledge the sulphur/yellow metal issue and claim it's a thing of the past. They say they mitigate the problem with other additives and by substituting a different, less reactive type of sulphur - inactive sulphur.
If you're still wary of yellow metal damage despite the assurances, a list of potentially harmful products, by descriptions, is here.
In 2018 I tried
Red Line 58204 Heavy ShockProof Gear Oil
In 2019 I switched to
Ravenol J1C1133 SAE 80w-90 MZG Gear Oil API GL-4
Red Line is supposed to be yellow metal safe. Ravenol states their product is "Neutral to metals and sealants used in transmissions and differentials".
Difficult to compare from one season to the next, and clutch adjustment makes such a difference, but the Red Line seemed to make shifting down to first more crunchy than with the Ravenol.
Draining the Ravenol post-2019 season the oil appeared quite clear and clean after the gray settled out. The amount of 'metal' seemed normal to me.
I'm using the Ravenol GL4 product again here in 2020 and I'll probably continue using it as long as I find it available.
However, if ever faced again with the prospect of using a GL5 spec gear oil in the Bonny, I will try not to be overly concerned.