I've had the chance to talk with a Stil factory rep about gas.
Absolutely never, ever, ever run E15 in your chainsaw (the shop I take my Husky to says the same thing). That appears to be the universal recommendation from every small-engine (including mower) on the market. He stated it would be better to get alcohol free gas then the regular gas/alcohol blend.
He also recommends 89 octane or "mid grade", he strongly recommended against 85.
What goes "bad" in today's "gas" is the various additives put in gas.
Both people also recommend never keeping gas past 90 days, even with fuel stabilizers.
Oh ya, use a good stabilizer.
Making sawdust mostly, sometimes I get something else, but that is more by accident then design.