Cadillac Repair: ac conversion, conversion fittings, ac conversion

Question
I get different opinions about converting from r12 to 134.  Kits are available to convert, some have an oil additive which is supposed to eliminate the need to vacuum out the old, some just have the conversion fittings and 134.  What is the downside of just adding 134 without additives or vacuuming?  Thanks

Answer
Well ive heard from a few people that you can just add 134 with the oil and itll mix fine with the 12, but ive read in my auto tech book not to ever combine the refrigerants. And R-12 performs much better than 134, if you need a charge i would go to a service shop or an a/c shop and have them recharge it.