I wanted to add to this thread what I did personally to my 91 Sedan. A/c
started getting warmer (then very warm= not working)this January. I researched the subject and found a company at
Maxi-frig refrigerant--Colder & Lower Head Pressure - Maxifrig that sells
a freon ($8.00 a can) that is compatible with both freon ref. oils. I changed the fittings to 134 fittings on the car ( as the can of freon comes with a 134 style top) and installed 2 cans of the freon. Has worked very fine since Feb. 15. I did not evacuate the system nor change anything other than gauge fittings. So far so good. The freon that I added does contain a leak dye, but I haven't seen where my leak is yet. I don't want to make any specific recommendations, just relay what my experience has been. If one tries this,
without using gauges, be very careful NOT to overfill the system. Good luck.