Gear Lube for mdl 80 diff??

[Al S]

My manual says 90W gear oil. I would like to use Amsoil synthetic, but not
sure if I should use the 75W-90 or 80W-90. Average year round temps are
75-94, so I don't need a lube for cold weather. I an leaning toward the
80W-90 because of the heat/freeway traffic, etc. but I don't want to
sacrifice any mileage.

TIA

Al Streicher
'90 80 Non-Q BLK
Mililani, HI

BTW, dealer shop charge $145 per hour.