I have a 502 low temp water cooled consdensing unit which is working fine but the water bill is huge. I would like to install a new remote an air cooled 404a condensing unit and hold onto the water cooled one in case of super high ambient conditions or just as a back up. Both units will never run at the same time. What gas should I use? Will having mineral oil and POE in the same system be a problem? Can and should I put POE into the old compressor? What will POE do to gaskets/TXV which have had mineral oil? Am I missing something?