I do not know whether this is the right place, but will ask.
When I bought the truck, from the service told me that i must drive at 2500-3000 rpm, for best performance and care to engine. However, I discovered that I could drive into town with 1000-1500 rpm at flat road or downhill (50 km/h; 5th gear). How bad could it be for the engine?
On the other hand, when driving at low rpm are all cooling systems work well or they are included only at high rpm?

I hope you understand my questions...