Wasn't there a similar thread posted on the rubbishness of these sensors???
Mine is also always hovering around the 70degC mark and has been for years not matter what I did (system flush, new thermostat, new hoses, new coolant). Rule of thumb is that oil temp is usually 10degC higher than water since most of the cooling is actually done by the oil.
According to Alfa's dials, the engine isn't even warm and I have had the sensor changed!!! I guess that the position for the sensor isn't very clever since it is right at the back of the engine on to of the sump where air may have some cooling effect.