I agree with 7 on this one. While I don't like seeing fine dust inside the saw there is always some to find. I do not like Stihl's filtration system on their older generation saws like the 440,460,660 and so I have changed to the Max Flow filters. They perform much better than OEM but still a little dust gets inside, especially cutting burned timber. My MS361 has the worst filter ever designed, leaks sawdust like a screen door, but the saw has not failed and shows no signs of undue wear.
This image is from 10 days ago. MS361 with a 22" bar, could not quite make the cuts touch. Cleaned the carb at lunch, airbox was a mess.