07-06-2013, 11:52 PM
Keep in mind that the valve clearance changes almost exclusively due to the valve seat wearing in, which reduces the clearance and tightens the valve. The valve face is beating the seat down causing the stem to get closer to the cam/bucket/shim. Choice of oil does not affect this at all. Materials used in the valve and seat make all the difference and the Genesis engines are first rate in this regard. Earlier Yamaha FZRs were terrible and many became difficult or impossible to start because the valves beat themselves into the seat so badly that they would stay partially open and the engine would not develop sufficient compression to fire up.