Adding 2L, that’s not low, it was empty! I assume you can change between gears no problem but it does the same thing no matter what gear it is?
First thing to do is adjust the clutch, on the right side engine (clutch) cover there is a nut, loosen it. Then turn the screw that is on the inside of the nut Counterclockwise until slight resistance is felt. Now turn the screw clockwise 1/8 turn. Then while holding the screw in place tighten the nut.
If that doesn’t solve your problem then it may be time to open it up but don’t fear, you shouldn’t have to go in too deep. It definitely sounds like a clutch problem which is behind the right side engine cover. I have had the clutch adjuster arm crack and cause the same symptoms that you described.
But before we go taking things apart, adjust that clutch.
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110