OK guys , Ive got 105k on my truck ( 08 DC 5.7ltr 4x4 ) . Ive been calling a few different Toyota dealerships to find out what there using . One of my local dealerships say they are using an oreilly brand 80W90 for $10.15 a pint that is not synthetic but they told me that a fully synthetic 75W90 is what came in the truck ( why would they change from the factory gear oil ? ) . Another local dealership says that 75W85 is what came in the truck and they could sell me 4 quarts of the " TOYOTA " brand gear oil for $37.51 a quart ( I dont think so ) . I called a few local parts stores in the area just to get a price on a 75W85 gear oil and no one carrys it , they say the closest they have is a 75W90 gear oil . Ive found a 75W90 gear oil at my local oreillys for $12.00 a quart in Castrol synthetic . Im kinda partial to Castrol since Ive never had any problems with it and have been running it in my vehicles for over 15 yrs now . What do yall think I should do and does anyone know exactly how much gear oil my rear end takes ? Also I have something hooked to the back of my Tundra way more weekends than not like a bass boat or a couple of 4 wheelers and if theyre not back there its usually something heavier so I want to run a good gear oil . Also , what do you guys think I should do with my front end diff ? I wasnt even gonna mess with it because I dont ride up and down the rd in 4wd , all of my 4 wheeling is done at under 25 MPH up and down the huntn club rds and honestly havent had to keep it in 4x4 very often .