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Hi Jon

While what you've said is true with regard to brass synchromesh pads in a manual gearbox, it's not true for the worm drive in a 4x6 bandsaw.

I've made a bit of a study of this 'yellow metal safe' topic. While I'm not an oil specialist, I am an industrial chemist and can sort through the chemistry of it.

Extreme Pressure (EP) additives were developed to protect steel hypoid gears in the differentials?of cars and trucks from sliding friction.??

Active sulphur and phosphorus compounds in the EP additive, bond to the gear surfaces to form a 'sacrificial film';?the higher the temperature, the thicker the film.? The anti-wear film can be produced at ambient temperatures just by rubbing two?metal surfaces together, but the film is much thinner than would be produced by rubbing at elevated temperatures.?

It works as 'anti-wear' because the film is weaker than the steel substrate.? It breaks, or wears, away under high load and then re-forms again afterwards, preserving the surface of the gear.? That same film also forms on any brass present, but the film is stronger than the brass.? It still breaks away, but it takes a bit of the surface of the brass with it, eroding the brass.?

Also, in the early days of EP additives, the active sulphur EP compounds broke down at elevated temperatures to form acids, which ate the zinc in the brass.

The corrosive potential of any oil is measured by ASTM D130, where a polished copper strip is immersed in the oil at elevated temperature (300?F (150?C) and above) for 3hrs and then the colour of the strip compared to the standard's colour chart.? A rating of '1B' (very slight tarnish) is considered acceptable for ‘yellow metal safe’, but the colour chart goes all the way to black! (a rating of '4C').

The amount and type of EP additives in a gear oil is denoted by its 'GL' number. GL4 gear oils have only about half of the level of EP additives of GL5 oils, so the? thickness, and bond between the sacrificial coating and the brass, is not as great as for GL5 gear oils.?If the bottle says "Exceeds requirements of GL4?and?GL5", it has GL5 levels of EP additives.?When measured, GL4 oils erode 2-4x less base metal away than GL5's.? ?

Also modern (>2020) GL5 oils have ‘deactivated’ or ‘buffered’ sulphur EP additives to stop acid forming, so technically the oil companies can say "there is no corrosion?(as in ‘chemical eating away’)?with GL5 oils".? However the sacrificial film erosion problem still exists.?

The reason GL5 oils will not corrode a 4x6's gearbox is that the 4x6 works at a temperature of only 60~100F (15~40C) where a car's gearbox operates close to 200F (95C). The operating temperatures are such that there is only a very thin sacrificial film.? So long as the EP additive is ‘deactivated’ or ‘buffered’, the 4x6 gearbox is safe, even at GL5 levels of EP additive. It's probably safe even if not buffered , but I've not tested it so wouldn't recommend it.

It's all very well to say this is what happens 'in theory', but what about 'in practice'.? Luckily?it is easily tested.?I have a piece of polished brass sitting in Lucas 75W-140 GL5 gear oil on top of my hot water cylinder, encased in polystyrene foam where it sits at a constant 37C (98.5F). It's been there since October 2019.?Testing?for corrosion?at the upper end of its temperature use range is a harsher test than it would see in real life.?

See for yourself in the pics attached, that after 4 years of effectively 'accelerated?testing', the part is still polished (the part that was in the oil is on the RHS - I'm holding the?bit that stuck out in the air). Although the picture doesn't show it, there is a very slight darkening of the polished surface over what it?was when it went in (shades of ASTM 1B rating?).? The brass gear will not corrode away in my lifetime!

Hopefully the 'yellow metal safe' oil myth is laid to rest insofar as the 4x6's gearbox is concerned.? However, if I had a car with a synchromesh gearbox, I'd be running it on GL4 - jv