Hi there,
Most all the R-12 substitues are a blend of butane and propane, in a percentage to produce approximately the same cooling effect as R-12. They work fine as long as there's no leak in the system. They are compatible with both mineral and PAG oils. (I don't have personal experience with the PAG oils, however, just repeating what I've read.) They are actually more efficient at cooling than R-12 is.
The problem is that the system MUST be labled that there is an R-12 substitute in the system. Failure to do so is against the law! Also, if there is a leak in the system, (and there almost ALWAYS is,) the percentage of butane and propane will change, and then the system won't work as designed after awhile when one of the gasses leaks out in a greater percentage than the other.
Interestingly enough, these gasses are used in many countries around the world instead of R-12, completely legally. They are environmentally friendly, even when leaked into the atmosphere. There is a slight danger of fire if they leak into the engine compartment in a large volume, but no worse than a can of hair spray.
If one uses them be aware that the liquid volume of these gasses is much different than R-12 - it takes much less to fill the system. If I remember correctly, it's on the order of 1/3 the quantity by weight.
Don't try to mix butane and propane to approximate the same thing, as the substitutes like Freeze-12 have been filtered and more water has been removed from the gases. Using regular butane and propane may rust out the AC system from the inside out.
I am personally aware of a Jag that has been using this stuff for 7 yrs now with no ill effects...
I'm sure that others will have plenty to comment on regarding this subject.
Regards.