I came across this today NU-22B Literature, and it looks promising for R22 replacement.
- 55% HFC-125
- 42% HFC-134a
- 3% Isobutane (HC-600a)
The literature goes on to further state that this drop-in R22 substitute is fully compatible with Mineral, AB, and POE oils, and that it is nonflammable to boot. So basically no oil substitution is required.
This ability to work with any oil, is made possible by the Isobutane part of the blend. I found it most interesting that such a small percentage of this component could make it work even with mineral oil. Of course keeping the percentage of Isobutane low, makes the overall blend nonflammable as well.
Could be a nice way to go in the first stage of a cascade, or even as part of the mixture for an autocascade. Of course it would also work well in a single stage unit.