There is a lot of "popular thoughts" on AC matters. Many of them are just pure fantasy..!!These kits you buy at stores may be dangerous due to lubricant compatibility. The effects of the damage would be long term damage as burnt compressor for oil starvation, corroded evaporator, etc. What many guys doesn't know is that to completelly change the system for one called 134a capable, seals must be changed all over, including those well inside the vehicle like in the expansion valve.
Here is my advise. Chenge the seals on hand and the receiver/dryer. Assemble your system, evacuate the air into it (vacuum pump) and charge it with a 134a can. Measure the pressure on the system, wait untill the next day (same outside temp) and check for pressure changes(this is if you don't have an electronic leak detector). If the pressure came down you got a serois leak if not, you may have a little leak or none. Get you a quart of an oil called Polyalpha which is compatible with mineral oil and PAG. Charge the system with 4 oz of oil and put 3 more cans of 134a. One of the major problems with oils compatible with 134a is moisture absortion, but this new oil (Polyalpha) doesn't.
After that charge check system operation and see how long that charge at $20.00 last. If it last less than a month you need to find the leak or keep on charging, your choice.!
That bolony of r-134 compatible compressors is because of the seals, but if the oil is PAG or POE, currently used on 134a. Other oils like Polyalpha do not have that problem and they are compatible with Buna-n and nitrile.

So, if you have the vacuuum pump and the suggested oil, go for it. If you need more info on AC system go to www.aircondition.com, then go to "questions" and there is a board like this one to answer AC topics. Do not trust too much the "street remedies".

Thanks BIMMER People! Anyway I need to fix my AC in an 89 735i. I just purchased a used AC compressor from a friend and I was wondering if anyone knows where I can buy a kit (fitting, tubes ect..) Also I'm amazed that my car holds 4.5lbs of freon?!!! Sorta spendy at $67 a pound. Anyway any help would be greatly appreciated.
David