The first number on oil is the viscosity when cold. As the transfer case never gets hot in an engine sense, the oil will never ride the temperature curve to the second number viscosity. The first number is the effective viscosity for the transfer case application. I would prefer running something like 20W50 for transfer case.
BTW: A trick with the NP203 with part-time kit installed.
The issue is that the rear output shaft bearing gets oiled by splash oil that runs down into some oilways in the case inside surfaces, then run down those oilways to the bearing. With the part-time kit, when in 2WD, there's no splash, so no oil delivery to the rear output shaft bearing. It doesn't need to be oiled constantly, but it needs to be kept wet or you will burn up the rear output shaft bearing, which is a major PITA to replace.
What I did was shift into 4WD every time I started the truck. The front Warn hubs were unlocked, so no issue. After about ten minutes, I would shift into 2WD. Since you are disengaging the splines, not engaging them, you can do that on the fly. This wetted the rear output shaft bearing every time I started the truck. On a long trip, I did the same thing whenever I had a rest stop or fuel stop.
Rusty
