To start off I live in a small town where everyone knows everyone, I am good friends with the salesman whom sold me the truck and he is great friends with the owner. I never get up sold or even try to, everyone at the dealership knows me etc.
I hit 100,000km and my rear diff fluid looks new and I wanted to change it, I went and purchased 75w140 as that’s what the manual states to use, I went over to visit the salesman (as he is a friend of ours) and my child poses for dealership pictures when the new owners don’t want to, it started as a joke and has taken off he asked if I called the dealer to check the price, I stated no because the dealer is always more expensive. He talked with the parts manager and they ran my vin, as per my vin my rear takes 75w90 and if my truck was in getting work done and they needed to add fluid it would be 75w90. So this got me to digging, the same owner owns a tirecraft (a tire shop that also does mechanic work) and has their own software for instructions on fluids, mechanical work, the time jobs should take and the amounts of fluids needed in all different makes and models. According to their system by inputting my make and model it tells them to use 75w90 and 2.2 litres.
Now I am confused, the manual calls for 75w140 but all other places I’ve contacted including the dealer call for 75w90 I don’t have the limited slip and just the “conventional rear end” 3.21 gears which fluid should I use? Or just leave it alone?