Volkswagen Repair: VW Jetta Oil Sensor light, vw jetta, weight oil

Question
I have a 1995 VW Jetta III.  

The problem: after 15-20 minutes of driving the oil light on the dash board activates (flashing red symbol accompanied by the warning sound).  When I rev the engine at above 2500 rpms the light goes out.  I regularly check the oil and it is not low.

Question: Why is the oil warning light being activated when there is sufficient oil in the engine and how do I fix the problem?  

Answer
Well, it can be one of 3 things .  The sensor is working just letting you know the engine is wearing as the oil pressure is too low, the thermostat is faulty.
 Does your car get really hot?  Get a lower temp. thermostat.
 What weight of oil do you use?  With any oil, as it heats up it gets thinner and depending on the engine wear(mileage) it cannot maintain the necessary oil pressure to stay in the safe zone.  Go to a higher weight oil like 10W40 or 20W50.   It'll kill the light, just be a little more difficult to start in the dead of winter with -50 outside.