2001 Dodge Durango 5.9 4x4
My manual says to use 80-90w for the rear end and for the 4x4. I've never seen 80-90w at any auto store I've been to. Would 75-90w be okay for the fluid? And does anybody know how much fluid each need?

 

I use Mobil-1 LS 75W-90 for my rear differential. I can't remember if it took 2 or 3 bottles to fill. Also, you'll need a bottle of friction modifier if you have a limited slip differential.

 

Interesting your manual recommends using 80W-90 in the rear end. My 3500 takes 80W-90 in the rear and keep in mine IIRC, there is no synthetic and is usually a heavier duty use. Since you either have the Chrysler 8.5 or 9.5, I'd recommend just using 75W-90 because it's been proven to work in that rear end and like samnbensdad said, be sure to use a Limited Slip friction modifier (Mopar is the highest recommended).

As for the gear oil, I'd recommend Mobil 1 synthetic because it's been proven time after time to last longer and stand up to wear much better than some others (although some do swear by Amsoil also) and you can go a little longer intervals without having to replace it as often.

 

the rear ends take 2.25 or 2.5 quarts, either way you have to buy 3 since they are only sold by the quart.

 

I've heard good things about Mobile 1 but I was thinking of using Royal Purple with the LSD additive already in it.

 

technically mobile 1 also has the additive in it already, but it is still best to put the correct modifier in there as even the manufacturers will tell you there is no one size fits all friction modifier

 

I would use Mobil One hands down because you are getting the same type base oil as Royal Purple but at 1/5 of the cost. That's why 70% of NASCAR uses Mobil......LOL


+1 on the use of the extra friction modifier even with a fluid that has it. Extra isn't going to hurt, but not enough will!........LOL