I purchased a new MS170 chainsaw from an authorized dealer. By buying a six-pack of Stihl oil I got an additional year's warranty. Seemed like a good deal so I did it.
Just past the one year mark, when the original warranty expired, the chainsaw stopped working. It was running fine when I put it up and would not start the next time I tried to use it. I took it to an authorized repair service and they found that the rings on the piston had disintegrated.
Just in case you think I probably don't take care of my tools, I follow the owner's manual maintenance schedule just as it is written, and I get my chains sharpened when they aren't cutting well anymore.
The repair service contacted Stihl with my extended warranty information and Stihl said that "dirt got into the engine and ruined the piston". They refused to replace my chainsaw. Apparently anything that allows dirt into a Stihl chainsaw engine is the consumer's fault. Couldn't be Stihl's design that lets that happen, could it?
So, my advice is treat a Stihl chainsaw as a disposable tool. If it lasts a year, good for you. If it lasts longer, even better. If it lasts less than a year you can waste your time trying to get Stihl to honor their warranty, but my experience says they will not.
DO NOT BUY THEIR OIL FOR THE EXTENDED WARRANTY! Their warranty means nothing and the extended warranty is only a means of getting a few extra bucks profit from you when you buy their oil.
From now on I am buying a Poulan at Walmart and getting both the bar oil and engine oil there, too. When the saw stops working I will throw it on the recycling pile and go get another one at Walmart. Over the life of three saws I will save enough at Walmart to get a fourth one free. Stihl doesn't cut that much better than Poulan anyway.
Monetary Loss: $175.