Take the Manual and show him in the Manual It says "Use Premium Branded unleaded gasoline with a Minimum octane rating of 89 RON"
then it has a blurb warning about using leaded fuel in models with catylitic converters,,,
then goes on to warn against fuels with lower octane may cause preignition (causing "pinging") which is accompanied by an increase in operating temperature,, which in turn increases the risk of piston seizure and damage to the engine,,,,
If he doesnt get right then there is a place on the Stihl website where you need to report him,,,
is it possible you flooded it a few times and have gas fouled the plug???? in three years if had to work on two,,, one had a combined and exact same problem with the operator choking the saw too much when trying to start and using regular 87 octane fuel,,,,,, I dumped the fuel put a new plug in it and it cranked on the second pull now it smoked like the dickens until all the old gas fouling was scavenged out of the crankcase and muffler,,,
The other had a busted fuel line between the tank and the bottom of the carb/airfilter housing!!!!!!!!!!!
I dont get it about this dude you describe as A dealer?????