You need to have an oil pressure test done on the car. This is not something to mess around with. The 1.8 engines had a sludge problem. They went to a larger oil filter and synthetic oil. You could have a sludged up oil pump screen, faulty oil pump, or faulty oil sending switch. Anyway it goes, take the car to a dealer or trusted vw mechanic and have the oil pressure test done. Failure to do so could result in engine wear, bearing failure, and finally engine failure. Check with the vw dealer to see if there was an extended warranty on your engine. The problem is, you have to show proof of all the oil changes.