A unique compressor system includes a compressor and an oil separation system. The oil separation system may include an oil separator element in fluid communication with a compressor discharge; an oil reservoir in fluid communication with the oil separator element that receives and accumulates separated oil; an oil-level controlled valve to control the flow of the oil from the oil reservoir to the compressor; and a check valve in fluid communication with the oil reservoir, the check valve being structured to prevent a reverse oil flow into the oil reservoir. A unique oil separation system, includes an oil separator element; an oil reservoir to accumulate separated oil; an oil-level controlled valve to control the flow of the from the oil reservoir; and a check valve to prevent a reverse oil flow into the oil reservoir.