300 cc (0,3 liters) sounds to low.
When you check the oil dipstick you need to place the scooter on centerstand on a horizontal surface. If the scooter tilts to side a little the reading on the dipstick will be wrong. Dont use the sidestand. If you use the sidestand all the oil will go over the the left side of the engine and you will not se any oil on the dipstick.
Do like this : place on centerstand on horizontal surface and remove dipstick, wipe oil of and insert dipstick in hole but dont turn it. Then read the oil level on dipstick.
Do you know if the people at the shop have been adjusting the valves ?
I would recomend using motorcycle oil instead of the car engine oil, there might not be any difference but just to make sure.
And why would the shop only fill it half way up the dipstick. They should have filled it up to the MAX mark and no more.
If there was a leak you would have seen some oil drips on the floor but you say that issent the case.
So if there are no oil leaks the oil must have come out the exhaust.
Maby you dont see any smoke in the exhaust but it might come out to slow to see any smoke.
Or maby it mostly comes by the pistions rings at high RPM when you are looking ahead.
On my brand new Kymco Super 8 these was engine oil comming out here when the engine was running. From the brown ring.
It wassent much but everthing i made a stop a few drops of oil would come out underneath the variatorcover towards the back there is a hole at the bottom.
I must have seen the oil drops by now if that was the case but you can check by removing the variator cover.
Also when you drain the engineoil its best to run the scooter for a while before because then the oil is hot and you get more of the old oil out.
When you have ridden a long trip shut of the engine and wait for it to cool down and then remove the sparkplug and have a look at it.
If the sparkplug is oily then the pistion rings might be damaged.
If the sparkplug looks fine and there are no oilleaks i would say you are doing it wrong when using the dipstick or the oil comsumption is normal... or the engine i just a bit worn and have a little higher oil consumption then normal.