Chainsaw Repair

It does get used a good bit but it might sit for a month or two over the winter, gas is from last year also mixed with all stihl products. I always run super in my saws etc. I guess I should add some stabil to my cans.... I just dropped it off to have them go over it anyway, the saw has never been serviced in the 10 years I have had it. I has always been kinda hard to start the last few years when cold. So as of right now my new little saw wont be here until next week and the big saw is 10/12 days out... So now I am sawless... I am going to stop at home depot and get some wood blades for the sawzall so I can at least trim limbs off the logs in the meantime...
The guys at the service desk told me that it was probably not exhaust build up because I have always run stihl oil in the gas. He said that happens a lot using other brands with their saws.... Don't know if this is true or not? Figured he would know....
Stihl Dino oil in ( was it the orange bottle?) has been the worst for build up on screens from what I have seen.
My gas is from last fall, if I add stabil to it now will it be ok??