Gasgacinch (auto parts store) on a new case. Yamabond (motorcycle shop) on used case halves. Be sure to use sealer on the 6 large 12mm case studs before you put the bolts on. Also on the cylinder to case surfaces. A thin layer on the oil pump and cover, oil pressure relief plugs and the cam plug. I even use some on the alternator stand and fuel pump and riser surfaces for a clean, dry, leak free motor.