Hi, welcome to JA.
Here's the deal...you need to run the engine under the exact conditions which will cause the oil lamp to illumiante, than you need to install the guage and check the pressure...if the lamp only comes under cruise speeds...than the oil pressure guage needs to be installed while you test drive it. Oil pressure relief valve is part of the pump and is serviced together, and this may end up being a pump problem...which is still going to be a large repair job. Also, I don't recommend any type of flus under these conditions...since you do not know the conditions inside the engine...if there is sludge built up, it will break off in a flush and cuase further oil pressure problems if not imminent engine failure. If the bearings are worn, it would be knocking by now...so they could be scratched from the low oil pressure, but not damaged yet, but the only way to figuer that out is to remove them and inspect...but again, this may not even be an oil pressure concern, so we need to know if the pressure actually drops before speculating anything. Below is the oil pressure specification..
Below is the oil pressure switch removal procedure...also be sure the mechanic replaced this switch and not another.
Removing And Installing/Replacing Oil Pressure Switch (M54) Turn off ignition.Release oil filter cap so that engine oil from oil filter can flow back into oil pan.Remove intake filter housing.Installation:Resecure oil filter cap.Check engine oil level and top up with engine oil if necessary.
Version 1:Location: On base of oil-filter.NOTE: Figure shows power steering pump supply tank removed.Disconnect plug connection (2).Unscrew oil pressure switch.Tightening Torque, 27 Nm
Version 2:Location: On base of oil-filter.NOTE: Figure shows power steering pump supply tank removed.Disconnect plug connection (2).Unscrew oil pressure switch.Tightening Torque, 27 Nm
It is supposed to be somewhatn wet in there...but this is not a good method of determining if there is oil pressure. If it is getting oil though, pulling the cap on a running enigne should spatter some oil out.
Thanks for the accept, the oil pump is mechanical, and is accessed from under the oil pan...but the engine timming chain is also partially torn down...I don't want to scare you but the pump and labor can rach up to $1500, depending on the labor rate of the facility, if you have AWD (All Wheel Drive) or not, and is it has one or two timming chains...typically it's around $900. below are all the dealer labor times and part costs.
I really hope it isn't the main and rods, but the labor and parts cost is below. At dealer labor rates i'd estimate the job between $3000-$4000.