Control Oil Line

Locating supply oil piping within the return piping is fairly common in industry, it saves space in tight insatllations and helps regulate temperatures. The return piping is generally not fully flooded and there is an air space above the liquid level. Reservoirs are typically fit withan air scavenging system which puts the system under a slight negative air pressure to prevent oil migrating thru labrynth seals and helps air to seperate from the oil in the reservoir. If you have steam carry over thru the labrynth seals then moisture enters the piping and can cause corrosion as you describe in uncoated carbon steel piping. Stainless might offer more corrosion resistance as would coating the inside of piping. You have to select a proper coating carefully so the oil doesn't get under it and cause it to flake off which can cause real problems with filters. Also distillate is very corrosive in itself as distilled water likes to leach minerals out of whatever it contacts. Seek out advice from specialists in Steam Turbine systems on materials to use.
If you do not have an offline oil conditioning unit such as a centrifuge or vacuum dehydrator I would suggest investing in one and use it religiously.