The volts will drop a bit when the lights are on, this is normal! But if the volts go below 12v you have a problem with the alternator! I would be getting a multimeter and putting it directly on the battery, because the volts really shoul'nt get as high as 16v! I dont have any idea about the oil pressure gauge though! Not without looking anyway. Ummm, and you wont get any oil pressure if the engine is'nt running! If you do, you really do have a problem!