Had a stihl ms390 too. used it for a weekend of tree trimming then followed the storage instruction and stored it.
A year later brought it out to do my annual tree trimming after using the stihl pre mix fuel I started the chain saw it ran but had no power or r.p.m. After checking fuel,exhaust ,spark plug etc i took it into the shop. I was told and showed that the piston had started to melt..
and o ya even though it has approximately 12 hours of use on it, it was out of the 1 year warranty..they have me an estimate to repair it. I decided to use it for target practice lol, and bought an Echo.