Tapping valves that need adjustment will get louder as the engine warms. Valves that are loose result only in a loss of power, where valves that are too tight will burn at their seats. For that reason, some folks like to keep them somewhat loose.

Also, some clutch mechanisms can develop a harmless knock once they get a lot of hours.

 

Not all dealers are liars.Some are mechanically illiterate though,which yours sounds like.Many fourstrokes need the valves adjusted after break in.
If in fact he was a moneyhungry liar he could get more money out of you to adjust your valves than to change your "thin" oil.
In my opinion you need both done but I wouldn't take it to that guy.

88' 250R many extras