Air compressors come in two distinct forms, those that are lubricated with oil and those that are oil-free. Air compressors lubricated with oil are more commonly used in industrial settings such as an auto shop, where they are left running for extended periods. The oil lubrication prevents the machines from overheating. Over time, the air compressor's oil becomes thick and black, making it less efficient as a lubricant. If left unchanged, old oil can result in the air compressor overheating and becoming permanently damaged. Changing the oil on a regular schedule is the best way to maintain the air compressor.
Determine the date on which the air compressor's oil was last changed.
Locate the sight glass or dipstick that measures the amount of oil in the air compressor.
Observe the amount of oil in the air compressor through the sight glass or by lifting out its dipstick.
Compare the amount of oil seen on the sight glass or dipstick with the volume recommended in the air compressor's manual. If the machine contains less oil than is recommended by the manual, it needs more oil.
Estimate the number of hours the air compressor has been turned on and running since the date of its last oil change. If it has been running for 300 or more hours, then it needs new oil. If you rarely use the air compressor and do not know how many hours it has been operated since its last oil change, you should replace the oil at least once a year.