As @racefever states, this is really subjective and hard to answer. I will try to do my best though.
Most of this is going to be done by feel. Every vehicle, even those of the same make/model, will have a different RPM at which it will get the best gas mileage. What I can tell you about when to shift, though, is keep it in the highest gear without lugging the engine. This will keep the engine at its lowest RPM for whatever speed you are at. Drive the engine up to a given RPM and shift. Depending on the vehicle, this could be anywhere from 2500-4500rpm ... it's just something you are going to have to get used to. After a short period of time, you'll get muscle memory as to where to shift and you won't even notice you are doing it ... it will just come natural. Unless you are taking the engine to the redline, you aren't going to cause the engine any damage.
As far as oil pressure goes, remember that an engine will run just fine at idle. You will not drive at idle, therefore you should never have to worry about oil pressure as long as the engine is mechanically sound and there is enough oil in the sump. If anything happens where the low oil pressure light comes on, then you need to worry about it. Even engines with an oil pressure gauge will have an idiot light which comes on if the oil pressure is too low. As long as your brother has kept good maintenance on the truck, this really is nothing to even start worrying about.