In reply to your question, it will not harm your system. As the other gentleman has replied, so long as you did not have any leaks in your system, you'll be fine. The only concern when this refrigerant was developed (r-134), was the fact that is was used with Polyolester oil in the compressor, instead of mineral oil. Polyolester oil is synthetic, whereas the stock oil is not. When the r-134 gas was developed, they said to take extra precautions not to use it with mineral oil, or else it woult pretty much turn to jello,and block up the system. I've been experimenting with it since '96 (I'm a refrigeration tech for a major company) and have never experienced any problems. Another suitable refrigerant is R-414, which is claimed to be compatible with R-134 and R-12 systems both. This refrigerant has been on the market for years now. After some research, all R-414 is made of is 98% R-134 + 2% R-22. The R-22 is generally used in household air conditioners, ant I think that they only use it to stabilize the operating pressures of the system. So the way I see it, If you can use 414 in a 12 system, R-134 is no different. Hope this was helpful, Talk 2 u later.