Buly,
My 2 cents...
My water temp sensor is in the thermostat housing (thermostat has been
removed).  That is the location that represents the coolant temp after
making its way through the entire engine, prior to being cooled by the
radiator.  This makes sense to me as it tells me how close to boiling
I'm running.  I think the location next to the oil filter pad will give
you an abnormally high reading as it is located just after the coolant
passes the combustion sections of the engine.  My 20B came with that
port plugged.  It also seems like it will provide somewhat meaningless
information as the coolant is not done with its task until it exits the
engine. Oil temp is just the opposite situation.  As I understand it, the oil
temp limits are set for incoming oil, not max internal oil temps, so as
to keep the oil going to the bearings from getting too hot and damaging
the bearings.  As I understand it, the oil that is sprayed into the
rotors for cooling gets much hotter, but that's ok as long as you don't
exceed the oil's maximum temp capability (hard to do with synthetics).
So, you want to know the oil temp going into the engine so that you
don't exceed the oil temp limits.  For diagnostic purposes, I have
installed oil temp sensors at the engine outlet (front cover) and return
(oil filter adapter).  I can see how well my oil cooler is working at
various OAT and speeds.

Mark S.