I have used Castrol XXL (SAE 40) in my gearbox for many years as recommended in the 2006 Vintage Lagonda Handbook without any known issue.

I was recently reading Which Oils by Richard Michell which suggested that some engine oils can be a bit light in anti wear additives for some gearbox applications.

I was hence wondering as there has presumably been 20 years of oil development since the handbook was first published, what gearbox oils owners are currently using ?

For info only, Chat GPT recommends using an EP90 GL4 yellow metal safe lubricant but I would be somewhat wary at the moment of using AI to give guidance in this area.