any ideas why?

 

Your oil pressure will flucuate as the RPM changes. For example, it may be around 30-35 psi at idle, but jump to 60-70 during acceleration, and be right on 40 at highway cruising speed.

 

it is really high at start up and drops when warm but goes up and down on idle

 

Oil pump, main bearings or even(much less likely but I've seen it a few times) a faulty sensor. Put 15w40 in it. If it is the pump or main bearings, it'll stay at higher pressure.

 

Cold oil will have a higher oil pressure then warm oil. The pressure will remain high until the oil warms up then it will drop to around 40 at idle. When the oil is cold, it is thicker, so it creates more pressure as it flows throughout the system. When oil heats up, it thins out and the pressure stabilizes around 30-40 psi. Completely normal and you have nothing to worry about.

EDIT: I'm sorry, I must have read over the part where you said it jumps around AT idle, that surely isnt normal and I would start where mantisman said. Good luck