Sam Brown
The compressor is hermetically sealed and part of the sealed system — that includes many of its moving parts that are lubricated by compressor oil. So when refrigerant is in the compressor, it does mix some with the compressor oil. However, there are various measures in place that separate the oil from the refrigerant to keep any significant amount of it from escaping into the rest of the sealed system.
The problem with leaving the refrigerator on its side is that this oil can, over time, seep out of the compressor and into other parts of the system. Once enough of it collects, you have a compromised sealed system.