Hey Adz... IMO, your on the money, thinking 'oil pump' but it might not be that bad... When the front crank seal becomes old, it tends to let small amounts of air passed it, as it trys to pick up the oil from the sump. Just check out the front of your engine, just below your crank pulley and check for signs of oil...

To answer your question... The oil sender unit is right below the Alternator