I've got 45,000 miles on a toploader that was rebuilt by David Kee about 9 years ago. I have always used Mobil 1
synthetic gear
oil in the toploader. Although others have reported issues when shifting rapidly when using a
synthetic oil, I have had zero problems in this area. I originally tried the
synthetic oil as an experiment, but mainly for the wear-resistance it offers. I've used both the 75w-90 and the 75w-140 with equal positive results. And no leaks either.
I recently installed the original T-10 transmission back into the car. Curious as to how the toploader had fared I removed the top plate and inspected the gears.
They all looked new. I urge you to try Mobil 1. If you don't like it for whatever reason, just drain and refill with the conventional oil of your choice. No harm done.
I'm also using the Mobil 1 gear oil in the traction loc 9" rear end. No need for a friction modifier additive as the Mobil 1 gear oil comes with it.
Gear Lubricants
Z.