Promoting the Preservation and Use of the Postwar Morris Minor
https://board.mmoc.org.uk/
):In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of the softer metal.
An API GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive that would be in the API GL-5 product, so the bond with the metal surface inside the transmission is not as strong, and therefore can be peeled off without peeling a layer of soft metal. This means that the GL-4 product provides a little less extreme pressure protection than a GL-5 oil but less wear on synchromesh components of a transmission. When a GL-5 oil is used in a transmission with synchromesh it can create up to 4 times the amount of copper in a used oil analysis as that of a GL-4 product. Synchronisers will eventually wear to the point where they become ineffective.
Very interesting Berni, thank you.geoberni wrote: ↑Sun May 03, 2026 9:51 am Almost a year has passed since this Topic started, but I thought I'd add some info.
I've never really looked into what the difference was between GL4 and GL5.
Just what makes GL5 unsuitable, or less than optimal, for older gearing?
I found an explanation here:
https://penriteoil.com.au/knowledge-cen ... -oils/1036
The actual difference being (note American spelling of sulphur on the Aussie website
In normal operation, the sulfur/phosphorous additive forms a black sacrificial coating on the gears and anything it touches with a little pressure and temperature. As the gears turn, instead of wearing, the sacrificial coating of additives is peeled off or worn off. This is normal and acceptable in all steel gears. But when one or more of the surfaces is brass or another soft metal, the sacrificial coating is stronger than the base metal, and instead of just peeling off, it takes with it a few microns of the softer metal.
An API GL-4 gear oil of any given viscosity has about ½ of the level of sulfur/phosphorous additive that would be in the API GL-5 product, so the bond with the metal surface inside the transmission is not as strong, and therefore can be peeled off without peeling a layer of soft metal. This means that the GL-4 product provides a little less extreme pressure protection than a GL-5 oil but less wear on synchromesh components of a transmission. When a GL-5 oil is used in a transmission with synchromesh it can create up to 4 times the amount of copper in a used oil analysis as that of a GL-4 product. Synchronisers will eventually wear to the point where they become ineffective.