Not sure about the YZ but It's a funny plug in the KX (NGK R6918B-8) which is is for racing and is about $30. I run a BR8ES which is $5 and dosen't oil up as bad. You could also try going down on size on the PilotThank you
About the tight technical stuff, the mechanic at the dealer told me I’ll be going thru spark plugs if I don’t race it like it was meant to. He said it should be treated like a chainsaw, high rpm and don’t let it idle for too long