It is all about testing. I always start with 40wt and go from there. If I notice after a jump that it bounces up like the tires are forcing it up, then I will either go down on the weight of the shock oil, or I will go with a larger diameter hole in the shock piston. If the vehicle is bottoming out, then I will either go with heavier oil, smaller holes or even stiffer springs. It really just depends on how it responds.
Now this is not the only factors to consider for shock oil. Transitions going into a turn, transitions in the center of a turn and transitions exiting the turn also play a huge role in oil selection.
But for a beater, just go with 40wt and stick with a middle size on the pistons and you should be golden.