I have a 2004 rev3 250cc, how much engine oil should I add or is there a certain level on the inspection window o should be aiming for? Also the other day I could hear a sparking/ arking sound, after looking I could see sparks coming from the top of the coil where there is a connector.  Any ideas what is wrong and what I need to do to sort it?

Beta recommend 550 ml oil I use putoline light gear oil and use 500 ml that way you get 2 oil changes per bottle 

Sound like your coil is either dirty, wet or cracked.   If the spark is going to the outside of the coil, instead of the plug, then your engine must be running very strangely if at all.

A new coil would fix the problem, but you could try drying the old one in a hot place for a few hours/days.