G'day Mate
First off, if the oil content was Coolpower with no Castor, it would not be gummed up, so I suspect Castor in there somewhere, & what % oil are we talking.
2, did you run the motor dry after last use, & put after run oil in it?, I think not!
3, 10% nitro will not need a colder plug, only when you get to 20 or 30%.
4, a OS A3 is a hotter plug than a #8.
http://www.os-engines.co.jp/english/...g/pluindex.htm
5, you probably have been running the motor too lean, causing very high temps, which will destroy plugs, or you have an issue inside the motor, ie, bearings failing, causing plugs to fail.
6, Is it possible that your fuel could be contaminated?
You stated that it had been 18 months since last use.
Try some new FRESH fuel & see if that helps.