i've mixed my amsoil sabre at 50:1 to start this year, because I just got a bunch of free old equipment, and want to slowly make my way up to 100:1 like I used last year. My units are running fine, just a tad bit smokey with the 50:1 sabre.
I highly doubt it was the oil. Might be a plug set at the wrong gap, or the Stihl Dealer didn't PDI and set the carburetor mixtures when they sold it to you. All stihl dealers are required to warm up the saw, and set the carburetor mixtures, as well as using a proximity tachometer to measure how the engine is running at idle and full speed.