Aircondition 1990. do you really have to have refitted to use 134a. or can you just put it in.

I have gotten away with it with a 1992 Mustang. Some people will tell you that you have to replace the seals in the whole system but you may not need to worry about it for as long as you own the car. Just replace them if you need to down the road and save your money now.
There is a difference in the weight of the oil in the different types of refrigerant. The converted systems never seem to cool as well as the old refrigerant but once your car is cooled then it stays cool in most cases.
I had my Mustang for 5 years after the conversion and it worked up until the day I sold it. I live in FL