"I'm inclined to drain a couple gallons and refill just to have strictly atf in there." - VernySanders.
Yeah. Lucus could have raised the viscosity too high. Cold in MI.
Hope to learn more by following this.
Lot of miles on that 4runner/Asin tranny 240,000
Good luck
Read recently of an Explorer transmission failing three times by the time the OP got to 150,000. Not Asin/Toyota brand
I had the dealer drain and fill at 100,000 miles. Now with 145,000 miles I don't want to touch it. Stopped by dealer today for some WS - they had none.
I will use Maxlife synthetic like you have done - but I don't know when as the tranny works so fine right now. Found a gallon of Maxlife (says Toyota on the jug) at $37. Another parts store want $58 dollars for a gallon of Castrol.
Temps where I am range from 60F to 100F - ALL YEAR LONG. Never any snow..
In Bahamas there are duties on importing everything for cars.
60% on tranny fluid, but shipping can double the cost of that VAVOLINE TO OVER $90 a gallon.
New cars have duty of 75% on the invoice.
No income taxes here, so it's real nice in that respect.