I have run everything from 20w-50 Castrol motor oil, Redline MTL, to Mobil 1 Synthetic. The difference in shifting is very noticeable between conventional 75-90 and Redline or Mobil 1. If I lived in a very cold climate, I’d be running ALL synthetics, they flow easier at cold temps. I’m running Mobil 1 now and shifts up and down are smoother than with the conventional 75-90 gear oil. I live in the Phoenix area and Redline and Mobil 1 did lower the temperatures on the tranny shifter and the t-case shifter when grabbed with my hand at the base of the shifters, especially during our 100+ summer months.
https://www.mobil.com/en/lubricants/for-personal-vehicles/our-products/products/mobil-1-syn-gear-lube-ls-75w-90/https://www.redlineoil.com/mtl-75w80-gl-4-gear-oilBy the way, in my rebuilt 5-speed, the tranny shop installed Toyota Factory synchros, and the shifts were a little notchy for about 1,000 miles. I switched from conventional 75-90 to Redline MTL (recommended by Redline Tech) and it really made the shifts much smoother.
Personally, I would NOT drain and refresh my tranny or t-case oil after a "break-in" period.
Gnarls.
