ATF fluid top off with wrong oil

Personally, based on my knowledge of what’s in the T-case, I would not worry about the 75/90. In a manual transmission, where there are many parts that require a specific weight and chemistry of gear oil to “feel” right, act right, and last a lifetime, the oil type and viscosity is very important. In a transfer case, which you shift once or twice a month, these issues largely disappear. The issue of high sulphur GL-5 vs medium sulphur GL-4 is often discussed, but that is largely only an issue for MTs where the bronze synchronizer rings are constantly engaged and disengaged.

My guess:

Toyota “LF” stands for “low friction”. The likely goal of using straight 75W in the transfer case is minute gains in fuel economy as the oil will thin out at running temperatures. Either that or Borg Warner buys cheap 75w oil by the train-full. Who knows?

 

People might call me out for this, but I at 5k miles I replaced all the fluids in my truck. At the time thinking 75w spec fluid was a typo I put 75w90 in the transfer case and 35k later everything seems fine. No issues yet, though I can't really say much about long term useage of it. Toyota hasn't released the specs for 75w LF fluid so no one seems to know what it is for sure. Though it appears to be a thin fluid similar to transmission fluid, I say this because redline d6 atf meets specs for 75w fluid though not toyotas 75w LF. Also from experience ATF seems to be very popular in chain driven transfer cases in other brand vehicles