[arucano]

There can be several reasons for the temperature fluctuation (indication).
a) It can be the oil thermostat
b) It can be a faulty temperature sensor
c) It can be a cooling problem.
Since the M cars show only oil temperature on the cluster, you will should have a look at the engine cooling system and temperature. I think you can access the coolant temperature via the "secret" menu. I would check that first before you start digging into the engine. Engine temperature variation can result in a lower viscosity and a higher temperature of the oil.
Check for obstruction of the air flow to the oil cooler as well.