a. What does the manual say? I suspect that if the truck has a non-north american manual, it will even recommend that kind of weights.
b. What oils are availble to you? How long of drains? How long of trips? What year/motor/milage?
c. If I had to randomly pick an oil, without knowing anything more, I might go with a HDEO 15W40, or 15W50 Mobil 1. There is nothing wrong with Castrol 20w50 that I know of, but in the heavier grades of conventional oils, it is hard to beat the HDEO's.