* 75W-90: This is a common viscosity for rear differentials in trucks of that era. It might specify an API GL-5 rating.
* 75W-140: If you live in a very hot climate or tow heavy loads regularly, a 75W-140 might be specified. Again, likely with an API GL-5 rating.
Crucially: The owner's manual or the sticker on the differential will specify the *correct type*. Don't rely solely on this information; consult your vehicle's documentation. Using the incorrect gear oil can severely damage your differential.
The API GL-5 rating is important because it indicates the additive package suitable for hypoid gears, which are commonly found in truck axles. Using the wrong additive package can lead to premature wear.