in a direct injected motor you have o rings on the injectors. These o rings are made to seal with whatever fluid is supposed to run there. Other fluids may contain harmfull materials. The whole reason alcohol based fuel system cleaners are NO No's is that the alcohol will harm o rings and other rubber componants....
Unless you put a non detergent motor oil in, the motor oil contains chemicals that may react with the injectors and or o rings. This could casue premature failure of one or both of these items, resulting in a spendy bill. To boot, that oil your adding has a different wieght and viscosity level as diesel fuel, so it does not just "mix" with your fuel, it on the other hand gets "pockets" of oil while driving and all the oil will settle to one place when the truck is parked, thus not allowing any major benifits from it since it doesnt blend with the fuel evenly.....