There is only 1 main regulator. Replacing the Range/Oven regulator was probably not needed. WHY did they replace the Oven/Range regulator? Oil in a LP system does happen, but the oil can be seen. They should have showed you the old regulator with the brown oily film IF it was contaminated LP. IF the Oven regulator had OIL, then your system needs flushed(Expensive). Oil getting downstream is highly unusual. Usually just the Main regulator catches and stops oil intrusion. The simple answer is, the LP regulator failed and it just needed to be replaced, no Oil or anything else. Doug