I like to use an ester based oil with is compatible with both R-12 and R-134A. You will also need to change the receiver /drier to and R-134A compatible type. All new receiver/driers today are both R-12 and R-134 A compatible. If you don't do this the R-134A will bust the old desiccant bag in the drier and spread desiccant through out the system and you'll have a real mess. 15 inches of vacuum is no where low enough and some thing is not right some where. A good tight system will easily get down to 28 inches and hold that vacuum for many hours. You are not anywhere near being able to recharge your system and get good results when done. Good A/C work requires attention to detail, it's not hard to do but short cuts will only get you in expensive trouble.
