I basically just check mine once a year in my normal go through the whole machine maintenance. Both of my Zuks sound like a sewing machine in the valvetrain, which is basically perfect. Some engines are make more valvetrain noise than others. If their in spec, and nothing is worn, and you see no signs of exhaust smoke on start up pointing to valve stem seals needing attention you are good to go. I have chewed up cams, and heads in the past on Honda's that didn't have external oil cooler's, which your EX does. Without a doubt the best engine Honda has ever had,,in dirt bikes or the 400ex. Besides the 250r two stroke of course,,,lol!
05 Suzuki DR650, Mikuni TM40 flatslide pumper carb,JE 10.5:1 piston,stage 2#190 Webcam,open airbox, Twin Air filter,FMF Q4, FMF Powerbomb header,Pingel fuel valve,Ricor Racing rear shock, Ricor Racing fork Intiminators, Progressive springs, Sargeant seat,DRZ250 tailight,custom plate,LED's,GPS,sumo fender,PT bars,Cycra brushguards IMS lowered pegs, aluminum skid,bearing chain roller,Pirelli's,Wolfman tank bag(gone)
06 Suzuki KQ700, Kenda Bearclaw HTR radials, Warn RT25