Back when retrofits were all the go, i made up a short piece 1 1/8" pipe with a piece of 3/8" in one end with a sight glass and shraeder valve, put some mineral oil in vac out and charged a small amount of R134a into it. the two fluids never mixed and when shook up would go cloudy/milky. Nothing like seeing it for yourself. When we used to do a lot of retrofits we would sometimes find a yellow enamel type deposit on the sump plug, guys from castrol said it was sulpher from poorly refined mineral oil. We used to have a customer with a frigidare open drive that he ran on duckhams engine oil (green one) because the shaft seal leaked and the previous engineer had told him to use it as it was thicker! Can't remember how well it worked as i was six or seven at the time.
cheers Jon