Hi Florindo ..
With many scoots its not possible to check the oil level or state of it , besides feel it between the fingers and giving it a visual inspection .
As you propably dont know when the previous owner last changed the oil its better to just replace it for a new bottle .
(wont cost an arm and leg ;))
Bigger 'problem 'is how to get the old oil out .
Dunno if the NRG has a special oil release bolt on the carter , bike like a speedfight hasnt , theres just one bolt for filling and draining there. If it has a drain bolt its easy , just let the old oil drain and fill it up again.
Scoots like Peugeot you have to pull vertikal so its upright on the rear wheel .. hold the rear brake and pull her up , gravity will drain the old oil out then.
I know some people drain the old oil using a larger size hospital syringe with a smal rubber hose on the tip , and suck it out that way .
How much goes in I dont know exactly , maybe u can get that out of the service manual wich you can find in the manual section here .
Thanks for stopping by here at SH ..