Well, You have these choices: www.refrigerants.dupont.com to work with. ISCEON MO79 oil- MO,AB,POE or Suva R-404A oil- POE , Suva R-408A oil-AB,MO, Suva R-507, oil-POE Suva HP-80 oil-AB,MO. Have a Suva day.
R-408 is probably the easiest replacement. The manufacturer says it's OK with mineral oil, but I still think it's best to change part of it out with AB. Or...you could do a complete oil change to POE, which works well with 408, and down the road a ways you could do one more oil change and you'd be good for 404.
HP-80 is also a fine replacement for 502 low temp, use AB oil for that one.
The best long term solution might be 404. Go with all POE oil, you might need to change it twice to get the original oil content down below the recommended 5%. Change the dryer and pull a hard vacuum. POE has a tendency to dissolve all the built up crud in the system so I'd plan on going back in a day or two and changing the dryers again.
With either refrigerant, check the superheat when near temp. None of the replacement refrigerants are actual drop ins so you'll likely need to make some minor adjustments. Typically with 408 the TXV will need to be closed off a bit.
We had a tandem system with 7.5 hp pumps that had compressor failures at 2 different times, so we were retrofitting with a -10°F evaporator(couldn't shut down whole unit). We chose HP-80 and zerol 200TD oil, and one was done in 1993 and the other done in 1995, with absolutely no troubles as of today. Box is a 40' X 15' X 7' meat storage.