Possible Causes:

Low oil level
Dirty heat exchanger
Faulty thermal bypass valve
Plugged oil filter
Plugged air/oil separator
Obstruction in oil system

Possible Causes:

Clogged separator scavenger line
Incorrectly adjusted scavenger line
Oil sump overfilled
Oil foaming in sump

Possible Causes:

Plugged air/oil separator
Airend bearings starting to fail
Obstruction in air lines
Obstruction in oil lines
Dirty or spent lubricant

Possible Causes:

Compressor temperature too hot
Ingestion of foreign substance through intake
Excessive condensate in system
Mixed oil types
Incorrect oil additives

Possible Causes:

Faulty inlet valve
Plugged air/oil separator
Plugged intake filter
Excessive leaks in plant piping