Vilter 440 NH3 oil loss

I have several Vilter 440 compressors for a Nh3 system and one of them is using way to much oil and seems to be running a little higher in temp (oil) than normal. Any suggestions. Also what would cause the oil to foam up? That is a seperate issue with another compressor.

1/ could be oil returnor internal relief blasting hot gas into sump or brocken suction or discharge valves . Also worn piston rings or scuffed cylinder liners .

When was machine last fully serviced ?

2/ could be liquid ammonia in oil from flood back.