BMW: BAD OIL PUMP?

Hi group:
I recently bought a 1991 535 i with 145,000 miles on it,the car runs
beautifully with plenty of power.However,a couple of weeks ago I
noticed that the "oil can icon" on the dash takes a lot of time going
out after I start the car,on Monday I got a message on the display:
"check engine oil pressure",I have taken it to the local BMW mechanic
twice and they claim they cannot replicate the message.They it is
possible that there is a "bad pressure switch"triggering the code.
Now,I don't want to take a chance and blow the engine,should I assume
that with this mileage it's time to replace the oil pump??Also,while
I'm at it can anybody tell me the socket size to rotate the
cranckshaft in order to adjust the valves?Does this particular year
model need to have the valves adjusted at all??,Thanks in advance.