I was driving the van about 1 week ago down a major arterial, and
the oil pressure light/buzzer came on.  The van has about 99K miles.  The
short story is: oil level is ok and the engine is not overheated.  I heard
no knocking ( although now all I remember is the buzzer).  I parked the 
van asap and let it sit until I could setup a mechanic to look at it. 

There are warranty issues here, and the van was seen by 3 mechanics for
correct diagnosis and repair.  BTW, the light did come on when I took it
to M1 but not to either of the other mechanics.

M1 said " Oil switch 0.45 bar goes out.  ok.  1.8 bar stays open after the
engine is warmed up.  Changed 1.8 bar switch. light still on.  measured
oil pressure. Low! warm engine 0.45 -0.54 bar. It supposed to be over 2.0
bar".  M2 ran his own tests and said "oil pressure too low at 3000 rpm.
2.0 bar instead of 2.2 bar."  M1 and M2 said either the oil pump was bad,
or something more serious and expensive was wrong w/ the engine.  They
said there was nothing wrong w/ the sensor.  M3 ran his own tests and
said: "55 psi cold idle; 72 psi Acc[?]; warm to hot: 7-8 psi (1/2 bar)
idle, 30 psi at 3000 rpm, 40 psi at 4000 rpm".  M3 felt there was no need
to replace the oil pump or get into the engine; he could not reproduce the
problem.  He called a local dealer (Campbell-Nelson in Edmonds, WA) who
said they had torn down engines after having this complaint and found
nothing wrong.  He recommended only that I change to a heavier wt oil,
which I will do.  The oil in it now is 10-30, and I will use 20-50,
Castrol. 

In your collective wisdom (1) why did this happen?  (2) What, if anything,
is wrong w/ the oil pump or engine or sensor?  (3) What do I need to do now?