Air condition of chevrolet astro van problem !?

Well i have a tiny leak in the AC system and i did not find freon 12 so the dealer changed to freon134 without any replacements and he told me that he did the same thing before without any problems ,the AC gave a cool air FOR 2 months before it stoped again , now i find in the market freon134 with seal and i found also F12 and there is a gas leak detector in the market and AC stop leak that some freinds advice me to use it but the other asked me to leave it cause it will create other problems but both of them told me it is better to add a small quantity of compressor oil which i think it is better to do it by myself cause i will save each time more than $20 so i will buy gauges ,14oz of gaz and vacuum tool and compressor oil and will try to refill again but which gaz should i use?