Any way of determining which refrigerant oil is present in a converted R134a system?

The shop that did the conversion is no longer in business and the receipt fails to detail the lubricant. Additional components (hoses & R/D) now need to be replaced; which oil should be used to top off - PAG or ester? Or should the compressor be drained, the system flushed and an oil of known identity be installed? Esters supposedly have lower lubricity and are moisture sensitive, where as PAG is sensitive to existing chloride deposits in a retrofitted system. Is PAG preferred? TIA.