Loopeloose

Hi I've just bought a 2002wrx, the previous owner has fitted an oil temp gauge, on normal driving the oil temp is 99-103 which I'm guessing is a bit high, I've noticed when I turn anything electrical on (lights,radio,heater) the oil temp gauge goes up by between 5-10c, can anyone shed any light on why, has it just been wired in wrong, if so what's the correct way
Thanks in advance