I use Hotshot on 95% of my service calls on both R12 and even R134a systems, it works very well & It has a temperature vs pressure rate thats dead on to R12, I've never had any compatabilty issues with Mineral oil , or modern system oils. I've never had to add any additives to early mineral oil systems, but you must recover all of the old R12 & pull a good deep vacuum to remove any water vapor & residual R12 from the mineral oil. ( just as you should with any refrigerant) I have in the past added some poly-oil to mineral oil based compressors that seem to be pulling too much run current, to just help ease the friction on an older worn system, but it has nothing to due with the use of Hot-shot.
In a capillary tube system & most expansion valve systems it works just fine for high & medium temp units, you just have to put it in at the recomended percentage of the original charge (typically 80%, )
In a system that uses a flooded evaporator system ( such as an automotive AC system) it is not recomended due to the fact that it is a blended mix. & different components will evaporate off at their own boilling points, which causes pressure surges in the system which effect the bypass / feedback circut in some expansion/control valves,
Hot shots' web page has lots of good facts regarding this & other replacement refrigerants they offer. so be informed & google it up!
