Synthetic Gear Oil recommendation?

[Rick Waters]

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.

[Steve Simmons]

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

[stephen stierman]

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.