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Gear Oil & Yellow Metals Experiment

Gearbox Drama

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 but had used it that season because it was all I could find locally.

Yellow Metals

The yellow metals are bronze, copper, and brass. Older motorcycles commonly use bronze and copper bushes and thrust washers in their gearboxes and transmissions.

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.

Gearbox Oil and Sulphur

Sulphur is one of the main additives used in modern gear oil. Sulphur is added to form a protective coating and to increase slipperiness. The additional slipperiness is required by modern transmissions and gearboxes.

The problem is that the sulphur's "protective coating" dissolves yellow metals.

I had previously understood that gear oils conforming to GL1 or GL4 standards were ok for old bikes, but not GL5 or GL4/GL5 products due to their higher sulphur content.

A little Internet research confirms that GL5 products generally have significantly higher sulphur contents than GL4 products.

One might conclude, therefore, or at least acknowledge the possibility, that GL5's higher sulphur concentrations pose a hazard to yellow metal components in vintage motorcycle gearboxes.

Divided Opinions

When I posted my GL5 concerns on a classic bike forum they were met by skepticism on the part of quite a few listees whose opinions I respect. However, a search of the Britbike and TriumphRat vintage forums turned up some equally credible sources voicing concern over GL5 products.

And for what it's worth, I found a post from 2014 in another forum in which a supposed Lucas "product manager" advised that Lucas gear oil is NOT yellow metal friendly and should be avoided in their presence. 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 to think?

An Experiment

I had a couple of identical used bronze bushes laying around, as well as partially full containers of 2 different GL5 products, so I decided to experiment. I would immerse the bushes in the GL5 oils for an extended period of time and then compare before and after outside diameters.

GL4 and GL5 gear lubricants side-by-side

The before measurements.

Pouring the oils over the bushes I observed that while the Valvoline was uniformly the color of dark honey, 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 has demonstrated 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 just 24 hours ("Lubricant Testing 101: Copper Corrosion" Nye Lubricants on YouTube).

While the inside temperature where this experiment was stored rarely exceeded 30C., the trial ran for nearly two years.


In April 2020, just one month short of two years after immersing the bushes in the GL5 oils, I retrieved them and wiped them off. They were both so shiny! Especially the Lucas!

Now, the moment of truth! I put the micrometer to them, and... ? Exactly the same. No difference from the initial measurements.

As noted, the experiment lacked high temperature. Nevertheless, over a significant period of time, neither the Valvoline or the Lucas GL-5 gear oils had any discernible effect on the bushes other than cleaning off oxidation.

Some producers acknowledge the sulphur/yellow metal issue, but 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, here's a list of potentially harmful characteristics to look out for.

What to Use?

If you prefer to stick with GL-4 products, here are examples of products that should be ok:

I have tried Red Line 58204 and Ravenol J1C1133. Red Line is supposed to be yellow metal safe and 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 a big difference, but the Red Line seemed to make shifting down to first more crunchy than with the Ravenol.

I've been using Ravenol for several years. Periodic inspections have yet to reveal any problems.

But if ever faced again with the prospect of using a GL5 spec gear oil in the Bonnie, I will try not to be overly concerned.