Preffered oil is 10 w 30 motor oil , no friction modifiers. Auto trans fluid type F, or a brand oil designed for motorcycles with wet clutches.
Do not put more than 650ml in the box, too much oil will expand with heat, and may leak out all over the motor.
The worst case senario is if the breather becomes full of oil, it may overpressurise and force through a crank oil seal.
Do your sums, just less than a Quart is too much oil, 650ml is just a little over 2/3 of a Quart.