MG T-ABC

I'd like to put some sort of 85 - 140 synthetic oil in both the gearbox and rear axle of my TC. I know it only requires GL4, and also am aware that some modern oils can damage the yellow metal parts in older gearboxes.

Right now, the car has 85 - 140 Quaker State gear oil, but I'd really prefer to use a synthetic. Any one have knowledge of which brand(s) are safe for the TC gearbox?

Thanks in advance.

I've used Royal Purple in my rear ends for many years, but standard oil in the gearbox.

I have had a ton of old cars and been into many old gearboxes, all of them with unknown serivice histories and I have never seen any damage to yellow metal that I can attribute to GL-5 rated lubricants. I would not lose sleep over this.