What Wire Size & Type for Lennox Split-System AC?
Their specification of 12 AWG wire would be fine if the run was a lot shorter, and if you never foresaw upgrading to a bigger unit.   

It squeaks under the 16A limit for continuous loads on a 20A circuit.  

As such, I would use 10 AWG copper wire, as it is the industry standard for runs to an outside compressor unit (i.e. A bigger unit), and it will assure your electricity dollar goes to run the compressor rather than heating up 12 AWG wire. 

I would then use a 20A circuit breaker.   A 30A would be allowed on 10 AWG wire, however the instructions prohibit this and seem to advise a 20A or at maximum a 25A breaker.   Listing, labeling and instructions must be followed; the electrical code requires this.