what is the best gear oil to use ?

For the record.

Any 10w 30 motor oil, with no friction modifiers, ATF type F, or any oil designed for transmissions with wet clutches.

And as a bonus.

Only fill with 650ml of oil, no more, no less.

No need to ask again!
 
For the record.

Any 10w 30 motor oil, with no friction modifiers, ATF type F, or any oil designed for transmissions with wet clutches.

And as a bonus.

Only fill with 650ml of oil, no more, no less.

No need to ask again!

Why 650ml? How would you know (no using glass window) how much to add when you dont know how much drained out?
 
Glass window tells lies.

Warm up the bike, drain all old oil out, leaving about 30 mins to fully drain.

Measure out 650 ml of oil and pour into filler.

Job done.