top off R22
 

Stash of R22 about gone.

Figure to top off from now on when needed with R290 (or BBQ)

Will just fill for 10 deg superheat, do not forsee any problems or other complexity.

Overlooking anything? (BBQ would be run thru filter dryer first)

Note: already figured that air fuel mix would remain below ignition even with 100% leak.

No need to give you a disclaimer here: I already know that you are seasoned and safe. This will work great for most residential systems that have a critical (small) charge. The PT chart will shift slightly, but the main thing to consider is oil control. R22 and oil are not highly miscible, but oil and R290 dissolve in each other well. So with any storage vessels (receivers, accumulators, etc.) the oil and propane mixture will float atop the R22.

Another recommendation by the experts is that since R290 has higher heat capacity, a few extra degrees of super heat is a good way to protect against oil foaming and liquid slugging. A lot of the refrigeration equipment being produced abroad runs off R290 now, and these units all seem to be undercharged.