2004 Volkswagen Passat Oil light and stop indicator light

I just had my 04 passat serviced with an oil change, new oil filter, gas filter, new valve cover gasket (was leaking into plugs) and new plugs. The oil light comes on sometimes at 2500 rpm's and then goes off. The pressure reads 25 pounds at 2500 and should be 29 pounds. The mechanic thinks that this could be caused by the screen on the oil pump from a build up of tarnish. My question is, could this happen because we are using a thicker oil now because I would run out at the 3000 mile mark before the next oil change, or could it be a sensor?