We have CAFE fuel economy regulations to thank for many things, like 0W20 oil and auto start/stop buttons.
I suspect 75W only exists for a marginal fuel economy improvement over 75W90. If Toyota can save 0.1 mpg on every truck/SUV running LG, that makes their CAFE rating go up. I also suspect that Toyota never cared much about the availability of the LG fluid for actual owners, after all the EPA fuel economy rating that CAFE is based is probably established based on the as-delivered vehicle. If the factory TC fill is LG and it gets 0.1 mpg better in the EPA test, that may have been the end goal of LG fluid. If Toyota cared what the 2nd TC fill after the LG is, they would promote their own lubricants (pre- and post-Iranian conflict) through their dealers and make them widely available for purchase. But they have not done that. Again, lots of speculation on my part, but Toyota shares little info so we are left to speculate

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Luckily my GX470 specs GL5 75W90 for it's Torsen center diff, so that is what I run.