A MD82 engine oil quantity was noted decreasing at cruise; so while descending to the destination an emergency was declared followed by a normal landing with zero oil quantity but normal pressure and temperature.

Narrative

During cruise at FL290 the First Officer exclaimed that he thought we had a problem. He stated that he had noticed that the right engine oil quantity had dropped from 10 quarts to 8 quarts. I began to monitor the right engine oil quantity and indeed it was decreasing at a steady but not rapid rate. I considered landing at a nearby airport which was right behind us but decided that we should be able to make our planned destination if we descended and operated the right engine at idle. We descended to FL240; and reviewed the engine failure precautionary shutdown checklist (but I did not shut down the right engine). We also reviewed the single engine landing and go around checklist and started the APU. The oil quantity continued to decrease and I decided to declare an emergency so we would get priority handling into our destination. We also requested Airport Rescue and Fire Fighters (ARFF) just for precaution. Oil pressure and oil temperature remained in the normal range throughout the flight so I kept the engine running at idle. I briefed the flight attendants on interphone and made a PA to the passengers so they would not be alarmed by emergency vehicles around the aircraft after we landed. We followed QRH procedures and landed without incident. Once we taxied clear of the runway; ARFF took a look at our right engine and confirmed lots of residual oil leaking from it but no smoke or fire. Oil quantity indication on the right engine had decreased to zero by short final but we had normal oil pressure the entire time; so I do not believe the engine went completely dry. We taxied to the gate without incident.