A good retrofit requires several steps, some of which you can't do yourself....
It is important to vacuum out all of the old freon, and make sure that your system can retain the negative pressure for a period of time (no leaks). You must use pressure gauges on the low and high sides while retrofitting. Some cars require an adjustment in the pressure cycling switch, and some require new orifice tubes. The new oil needs to be compatible with your system.
Some cars require larger condenser fans.
It is tough to do a good job unless you are experienced. I would recommend taking it to a good car A/C shop and having the leak fixed, and then recharged with R-12 or retrofitted to R-134a there.
1999 540iA Sport
Former 93 SSEi Owner
Former 89 SE Owner