Will propane work in place of freon?

I have a 81 300sd that has a quick connector on the suction side and the original connector on the high side.
How would I know if the oil has been changed for 134 or has the 134 put in over the freon 12 oil?
I heard that propane will mix with either oil. So I open a can of worms here!

But if the F12 oil has not been removed and the f134 oil has been injected and mixed, then the chances of my compressor locking up?
It would be nice if people changing this stuff around, would put a sticker or a note in the glove box.

Tom