What kind of gear oil does a trac-lok 8.8 take
 

I bought mobile 1 75w-90 will that work or will I have to use some thing like 85w-90 in my traction lock 8.8 rear-end

I think the weight you use depends on how often you change the oil. I used 80w90 and some friction modifier in mine.

Gear oil weights
 

75w90 hypoid gear oil is just fine.

Use 4 oz. Friction Modifier if the oil you bought doesn't already have it mixed in (check the label).

How often should you change the oil?

Differential oil chnage frequency
 

Not often, unless it gets contaminated or you have a leak, of course.

I've changed mine about every 50,000 miles or so over a 116,000 mile lifespan and never had a problem, although I should do it a little more often. More frequent changes won't hurt a bit but aren't normally required.