...Toyota actuators can be a bit grumpy when cold. Dealers like to replace them and charge a fortune. They can be contact cleaned and resealed but some models hard to get to so you pull the xfer case to swap or service the solenoids. Has nothing to do with visc other than how slow they are when cold.
Since Toyota had a history with transfer case GL5 gear oils, I would either use the OE MPG LF 75w or snub my nose at MPG and fill it with GL5 75w85 or 75w90. I dont see any commonly available GL5 75w80's in the USA. Once the gear oil is warm, it cant be any worse than the 75w LF when cold.
I dont practice shift on the fly shenanigans. I think that it would be smart to anticipate the terrain and engage/disengage 4wdHi 2wdHi as needed from a stop. I'd wager that the thinner fluid might be quicker than thicker fluids....for the clueless driver shifting into 4wd at 50mph, especially if there is some transfer case synchronisation when engaging 4wdHi. Disengaging 4wd should be a non issue regardless of fluid.
The Ravenol is used by all the guys on the Toyota Tundra forums. I have it in my Tundra. Runs smoother that the Toyota factory fluid. Many who have went with 75W-85 or 75W-90 have had issues. The older Tundras would be fine, but in 2014 they went to a new transfer case that requires the 75W. I would use it only. As far as I know. the only fluid in that weight besides the Toyota fluid is Ravenol and Redline.
You don't need that in your TC. In warm places it probably won't hurt anything, in cold places, you might have some sluggishness when changing.I get a "clunk" when moving between 4L and 4H if the drivetrain isn't warmed up. Not something I do regularly, but in rare instances I have to make that shift before I have driven much. Is that the actuator grumpiness you refer to? When everything is warmed up, TC shifting is quiet and smooth.
My TC has not been touched yet, so presumably it has the OEM 75W LF in it. IF they are even using it at the factory, LOL.
I don't live or drive in snow country, so I only shift into/out of 4WD at low speeds on unpaved roads.
(Off-topic fun fact: my Wrangler manual recommended that I put the transmission in neutral BUT still be rolling at 2-3 MPH when shifting into/out of low range! This was a problem if I had already started a climb then realized I need low range. Actually, TC shifting was a problem all the time as the TC lever was hard to move. I'm quite happy I now have a dial instead.)
So is anyone going to talk me out of using the AMSOIL SVG 75W-90?