Oil in circulation (OIC) in a vehicle AC system is an index to demonstrate the system having a proper oil charge. A well lubricated compressor is key for AC system, as it relates to compressor durability. The challenge that climate control development engineering faces is how to correlate OIC from test to test (test stand to vehicle level etc.). In this study, a method to correlate vehicle AC system OIC from test-to-test, stand to vehicle is developed. The study found that oil charge amount ratio, refrigerant mass flow ratio, and vehicle engine speed ratio are the key factors to correlate the test results such as stand versus vehicle, vehicle test A versus test B etc. To further reflect compressor oil lubrication conditions, compressor compression index (polytropic number) is computed for the different testing, and the ratio of compression index is also introduced to the correlation. The method accuracy was validated by multiple vehicle/stand tests. The study indicated the predicted OIC is very close to the actual tested value. This method can help analyze the test data to explain why OIC numbers change with the same system and close testing conditions, thus help to control testing condition parameters from test to test and stand to vehicle. The varying OIC results between stand to vehicle often result in running more OIC testing and consuming resources to obtain similar results for the sake of confidence. The method introduced in this paper provides a way to dissect the OIC relationship between vehicle and stand testing results, thus helping to make an intellectual decision on the OIC data.