Air Carrier pilot reported a fume event during descent. After collaborating with maintenance and further troubleshooting it was decided to ferry the aircraft to a company maintenance base.

Narrative

This report is being submitted only because it is a recommended step by ALPA: no errors in policies or procedures were made by the crew. Airplane was a Routine Overnight (RON) from the previous flight. Upon arrival at report time; the cabin was being cooled by ground air and was comfortable. We started the APU shortly before departure time. Climb and cruise were normal. Passing 11;000 ft. on the descent; the FO (First Officer) and I both noticed a strong (4 out of 5) 'dirty sock' odor coming from the cockpit vents. I turned off PACK1 and the odor began to abate. I called the cabin to ask the Inflight Crew if they were experiencing the same in the cabin. They said they were not. Maintenance was requested during taxi in; PACK1 remained off. Upon inspection of the tail section; I saw fresh oil coming from the APU service door seams and drains. According to the logbook; the APU had been serviced after a 'LOW OIL' message three days earlier. The Contract Maintenance Technician opened the APU service doors and observed oil throughout the compartment; including on the breather port. When he opened the APU oil service point; he estimated that more than a quart of oil spilled out and the reservoir gauge still showed over-filled. Maintenance Control had the Technician run both packs for 15 minutes at a high temp setting to decontaminate the PACKs with the intention to put the APU on MEL and have us fly our return flight with passengers. Inflight were encouraged to leave the aircraft for this procedure. At the beginning of this procedure; the Maintenance Technician also smelled the odor and agreed that the system was contaminated. The FO and I decided that we would do a brief engine run of the #2 engine at the gate before we boarded crew and customers in case contamination of the environmental system was still a problem. PACK1 was turned on first and did not seem to still be contaminated. When PACK2 was turned on with PACK1 off; we both smelled a strong 'dirty sock' odor again. I turned PACK2 off after about 10 seconds and we advised the Maintenance Technician. Operations determined that the aircraft would be repositioned to a Maintenance station. We ferried the aircraft unpressurized to ZZZ1.