What did you do to the plug? It looks new in the second photo.
You judge a plug by its color. Too white means lean. Too dark, means soot, probably is rich. Also, yours has a mild powder which is great. Too powdery is mean lean. I havent even looked at your insulator. I write this just judging the outside electrode. I hope I am right about all of this. It is what I think- so be disclaimed.
Also, as was said, don't change a thing....don't mess with success. I inherited a mower that ran fine, started first or second pull. Then I found a manual for it and tried to do it by the book and turned to junk. It said replace the spark plug like annually or every fifty hours....and I did. Spark plugs last a while. Look at yours- 10 years- barely worn and in great condition.
I had a Stihl 4mix FS90 that I ran the Ultra mix (silver bottle) with top tier 89 octane per the manual. It ran great. It did say dont use old fuel. But I only burned like half a gallon a year. So, I just cleaned the trimmer and topped it off at the end of the season and she did great come next year. The high end power did seem to suffer with stale fuel, but the effects werent permanent. When I poured in new fuel it ran perfect again, so its not like it clogged a jet.
On the other hand, I had a Craftsman mini 2 stroke tiller than I ran dry one year. I stored it for several years and it never ran correctly again. But the manual said to run dry. I suggest keeping OPE topped off if using a oil/gas mix. I dont know WHAT to do if something uses straight gas.
Anyway, back to the 89 octane/Stihl ultra mix. That half gallon I had left over I would use it the following year. I think I even made it part way into an additional season with it. So 2 1/2 years was great. It started and performed great. I kept it in an old red plastic can with a capped yellow spout and a capped yellow vent. It was is a very fine can and I think it helps keep fuel fresher. I even sat a five gallon bucket over it to help further.
For my FS90, I had for 3 years I just kept it clean, poured every last drop of mix into a one gallon can, bought exactly one gallon of gas to add to an empty clean can, shook.....add oil first the bottle says, I still have one but took the trimmer back. I never checked or even pulled the plug wire, I never checked the air filter. It was nice having something for once I wasnt always playing catch-up with (eg buying a new car and just following the owner's manual vs buying a barn find that needs constant restoration) They sold me a six pack when I got the trimmer. It performs well. Armor all products work well on the orange plastics. Keep the prime bulb clean so it doesnt dry rot. Keep the exhaust hole covered so nothing can enter when covered.
So, just put it back together, use the oil mix I mentioned, keep it clean and stored properly and let your grandkid worry about it (unless excessive use or dusty environment).
