what should the weigh of gear box oil be

Firstly, is the engine leaking oil?

Have you noticed excessive smoke, if so is it white and smelly.

If you loose oil the crankshaft oil seal could be leaking and also sucking air which can lead tithe death of a piston.

I could pay to leak test the engine.

Warm up the engine, drain all the old oil out and refill with 650 ml or either 10w30 engine oil with no friction modifiers or ATF type F.
 
drain it and measure it
maybe funnel it it to an empty bottle then read the side.
it only holds 650ml which is ruffly 3/4 of a quart says on side of bottle
if you overfill it ...it will spew out the crankcase vent tube routed by header exhaust.
you can witness it by running it looking at the sight glass should be between two notches running,then shut it off and it will fill sight glass.
but i would drain it/measure it to confirm it is loosing oil if you can visually see it.:)

10w30w is the manual recommends...i would shop at a dealer for it if you can.
only thing i dont like in my opinion is the 80w oils...they destroy your clutch..in my opiniono_O