I pulled the head last night on my '07 650R and noticed a thin even grey layer of gummy (not hardened) sealant below the head gasket. I'm almost certain some kind of sealant was applied by Kawasaki at the factory. Neither the Haynes or Kawasaki service manuals mention sealant when changing out the gasket. Anyone have a definitive answer as to whether a sealant is necessary? The picture below is after sealant residue was removed.