Oil gauge
You "can use the oil test port" , have fun doing it, on an 03 you have dismantle the EGR cooler to get at it, then you have to get a sending unit in there. Best way is to take off intake hose to turbo. Then take out the EOT sensor behind the Oil filter housing, "T" into it, have your sending unit pointing toward the CAC tube. There is just enough room to do it if you think ahead a bit and get some parts. not too many. It also does not rub or touch anything. Best of all she don't leak. As far as I know the oil pressure switch, the stock one, is just an idiot switch. Above 7psi the guage will switch to the " normal" zone. With 15w40 at start up I've had 100psi, and down to 12 psi at warm idle. 10w30 I have 75 psi start and 10 at idle. 65 and 52 psi respectively at highway speed.