Xavier:

With the engine cold, start the car and watch the temperature gauge while monitoring the upper radiator hose. The gauge should rise rather quickly to about 1/3 (8:30 position) within a few minutes, and the upper radiator hose should stay cool, then get hot all of sudden as the thermostat opens. This is normal behavior. If, on the other hand, the gauge rise very slowly to less than 1/3 and the upper hose warms up gradually but never gets really hot, your thermostat is probably stuck open and should be replaced. It would probably be prudent to flush the cooling system anyway, so this would be a good time to do the "stat".

WRT the oil, an engine flush followed by a change to a synth blend or full synthetic (especially if you're in a cold climate and do a lot of stop and go/short trip driving) would seem to be in order.