Unnecessarily using too high a viscosity wastes power and fuel. Many people like using Mobil 1 Turbo Diesel Truck 5W-40 and Shell ROTELLA T Synthetic 5W-40, which are readily available in the USA. I'll be using in a PC Duron-E 0W-40 in a Dodge 3500 6.7L and Esso XD-3 Extra 0W-30 in pretty much everything else. 15W-40 (dino) will generally be the cheapest oil to use.

ACCCC Engine Oil Article