You do realize that Navistar International manufactures a whole range of vehicles, and not just one, right?
As for what to use, it depends on your transmission. If you have an Eaton-Fuller transmission, then they recommend you use either full synthetic 50W oil or 80W mineral oil. I'd go with the 50W synthetic.
There are new chevy trucks that are coming out in 2015.
The 2015 Subaru WRX has a 6-speed manual.
The 2015 Mazda MAZDA6 has a 6-speed manual.
The 2015 Chrysler 200 has a 9-speed automatic.
The 2015 Volvo S60 has a 8-speed shiftable automatic.
The 2015 Chevrolet Suburban has a 6-speed shiftable automatic.
The 2015 Volvo XC60 has a 8-speed shiftable automatic.
The 2015 Chevrolet Tahoe has a 6-speed shiftable automatic.
The 2015 Kia Sorento has a 6-speed shiftable automatic.
The 2015 KIA K900 has a 8-speed shiftable automatic.
The 2015 Audi A3 has a 6-speed automated manual.
The 2015 Volvo XC70 has a 8-speed shiftable automatic.