I'm hoping someone in this group will take the time to answer this for me.

I'm in the middle of a course on HVAC in high-rises. The instructor has
stated that the oil in the HVAC system should be checked regularly for
quality and impurities. She says that if refrigerant is detected in the oil
then there is a problem. I was always thought that there is oil in the
system with the refrigerant and that this oil lubricated the compressor
etc.and one would expect to find refrigerant mixed in with it. I thought
that this was why you are always told to let a newly delivered fridge stand
for an hour or two before turning it on, to let the oil drain back down into
the compressor.

What gives?

tia

Peter H