I didn't want to start an new thread for this but I couldn't find a old one , so I bought my first truck around December of 2015 and I would like to change the differintal oil but I don't wanna stick just any 90 weight in it what do you guys prefer?

 

I just put any old 80W90 in it.... <_<

Seriously, if you want to go with a synthetic oil so you don't have to worry about changing it for a long time, go for it. Otherwise, just about any 80W90 oil will do just fine -- they all have to meet a spec to be labeled as such, which means they should all work about the same.
I think I went with Valvoline oil, it was whatever "name-brand" oil I found cheaply at Autozone.

Note, however, that I have an open differential, not a limited-slip setup. For a limited slip setup, you'll want to get an oil which is marketed for limited slip applications... which most are.

This page has a lot of info: Sterling 10.25/10.50 Fluid Replacement
That all being said... something I just found. Autozone's database said I needed 80W90, but that page I just linked says to use 80W140. I'm not sure which is correct, but everything looked in very good shape the last time I took it apart(no galling, shavings or unusual wear pattern), so I'm assuming 80W90 is fine.

Also... I've changed the oil in my rear diff twice in the last few years; First time, I cleaned everything really well and then used white silicone tub and tile caulk for a seal.
When I pulled the cover off last month... that silicone had worked just fine. No leaks at all, and I really had to pry the cover off.
At that point, I just grabbed a paper gasket at the store, because it would let me put oil back in it immediately(with silicone/RTV, you really need to let it set overnight before adding oil).
So far it's worked.

 

Thank you for all the information I have no idea how old the oil is so I know I'm going to change it but wanted to know what to put in but now knowing about those guidelines and stuff I don t have to be too particular on this job thanks again

 

80-90 gear oil works fine

75-90 mobil1 synthetic ++ mpg and 'forget it' change intervals

 

I run Amsoil in all my big trucks and little trucks. If you have a posi unit - you need to put an additive in or the Amsoil will slick up your posi clutches and you won't have a posi anymore.