That's the red cap Nylog, correct? I've used it on R-134a, R-12, and some blends I won't mention. Never experienced a problem. I say go for it.

There are synthetic a/c oils for R-12. Don't know that they offer any kind of improvement over good old mineral oil. The main advantage is they are compatible with both R-12 and R-134a, which permits a/c shops to stock only one oil.

From the DIY perspective you can run the system with R-134a for a couple of weeks/months to insure there are no slow leaks before risking $75 or more of R-12.

My $.02,

- JimY