I had my differential oil repl. like 4 years ago or 15000m ago(dont drive a lot in van) by lube it all.They charge me around $45.00 for it, they added LS to it as well, they used 75w90 but i m reading that i should use 80w90.What oil weight I should use and should i replace a gasket or just pour in thru plug hole.I doubt that lube it all replaced a gasket.What is difference between 75w90 and 80w90.

 

They might have gone with the 75w90 if you live in a northern state or because you only drive a few thousand miles a year. It runs slightly thinner and helps out in reducing wear while improving gas mileage in the colder months. At 15K you're only about 1/2 way before having to change it again, so I'd leave just it alone.

 

It leaked out i need to add some but i m trying to figure out what to use 75w90 or 80w90.

 

Use synthetic because at the rate you drive the van you'll never have to replace it again.

Now how did the fluid leak out in the first place? Because that leak will have to be fixed before you pour any fluid in there.

 

I think it leaked out thru gasket so i m gona replace that as well.

 

You're getting all kinds of worried about nothing. It's like if you used 5w30 instead of 10w30. not going to amount to any issue what so ever.

But if you really want to be super safe, go with a Pure Synthetic gear oil with the EXACT specification

Oh and 'generally' it is the recommneded fluid. If you were in a super hot climate and were using the vehicle in extreme conditions, it might be a good idea to use 90w140.