Firstly the oil shud be 0.7 litres for a 70cc motorcycle. If you notice a oil reduction between oil changes then it cud be either of two things. You are either burning oil or leaking oil. If you cant find oil stains where you park your bike then Sir you are burning oil.
Burning oil can be a as seriously problem as a worn out piston ring to be as small as lean air fuel mixture. Waisay before you start to blame the bike, why dont you check the authenticity and grade of the oil which you are using. Dont buy anything fancy.......just buy regular locally packed 20W-50 motor oil. I use Shell because because I can check its autheticity by confirming it with the scratch code provided.
For a change...... try a differrent mechanic......maybe your bike is running on lean air fuel mixture which can also cause oil to evaporate. There can be many reasons why your oil is evaporating, I told you the one I think are the usual culprits in Pakistan. Lets c what others have to say?