If the dealer said that 75w85 gear oil is the fluid for the transfer case, then he is correct. I did not look it up when posting, just went off recollection. However,I just checked at the ToyotaNation forum for Highlander and it does take 75w85 (or 75w90 synthetic) get oil. They are mechanical twins.
FYI the difference between 75w90 and 75w85 is that the 75w85 is very slightly thinner at high temperatures, but not enough to make any difference that could be noticed. Also it is easier to find 75w90 in stores and it is synthetic, which is a big improvement.
FYI Exxon/Mobil makes most if not all of the oil, transmission fluids, geat oils used by Toyota/Lexus. Although the products are made to Toyota/Lexus specifications, they are not that much different from the stuff labeled Mobil 1.

You can register here and access the owners manual online, also look up your car service history https://drivers.lexus.com/lexusdrivers/