Refrigerant will migrate to a compressor's crankcase because of a difference in ?????? between?

If the outdoor temp is below the saturation temperature (which will vary depending on type of refrigerant and current pressure) the refrigerant will condense into a liquid and the oil in the compressor will float on top of the liquid refrigerant. When the compressor turns on all the oil will exit the compressor and could cause damage until the oil goes through the entire circuit and comes back to the compressor.