Both diffs, tranny fluid, and transfer case once per year.

 

I like the Mobil1 75w90, but Amsoil is better, just costs more. But really M1, Amsoil, even Valvoline Synpower is going to be far better than dyno 90wt. But this isn't a synthetic is better thread, so I'll just say I run M1 in all my diffs & t-case, on the Runner as well as Land Cruiser. Change them every couple years, or when I'm doing other service.. probably ends up being 10-15K mile intervals.

 

I run various types of Redline oil in my drivetrain:
- http://www.4crawler.com/4x4/index.sh...ntheticGearOil

and I never routinely replace the oil. I do run extended breathers and don't do any deep mud or and only infrequest water crossings. I do find that I do have to drain the oil periodically for some other reason every 3-5 years. Changing gears/lockers/axles or whatever, so I will change the oil at that time. I think I have around 60K miles on the rear axle oil (filled in 2000), last of the parts filled at that time. Since that time, I upgraded the front axle and had a part break in the dual t-case (unrelated to oil) and thus had the t-case drained and while it was out, I drained and pulled to tranny to inspect the clutch. I'll be putting a wider rear axle on in the near future so that oil will get changed then. I do check the oil periodically and if it did look bad, I would crain and refill it.

 

Well thanks guys I'm going to go with the shockproof in the differentials

 

Every 15k which is about every 2 years