gixergav, your wheels are awsome....
whiteakita, to change the oil and filter there is the oil drain on right side bottom of engine, it requires a 13mm socket and a ratchet, the filter is also on the right side just behind the cylinder, its cover is about 2" around and comes off by removing the two allen head screws. one tip to remove the cap would be to use a bolt and thread it into the hole in the center of the cap, this will give you leverage and usually just wiggling and pulling the bolt and the cover will pop out, to get the cover out you will also need to remove the clutch cable from the lever just to get better access. if you look at gicergav's picture below you can see the red cover over the oil filter just behind the bottom of the cylinder, the drain plug is between the stator cover and the front sprocket.
i change the oil and filter every 7-10 hours but i also change the oil every 2-4 hours, i wouldnt recomend going past 5 on an oil change, then just change the filter every other oil change or a max of ten hours.
i use retella-t 15w-40 non-synthetic diesel engine oil along with a lot of other people, it is very cheap compared to synthetic 5w-50 and people have been using it long engough it can be fully trusted in my opinion, and its cheap enough you dont have to worry about 2-5 hours oil changes. these engines are good as long as the oil is kept clean and full. if it gets negelcted or low, things that make bad loud bang noises can happen
also i try to change the oil when engine is still warm because it will flow out better, and as a side note the easiest way to fill it IMO is to put the plug in and filter cover back on, then lean the bike against the wall or something on its right side so its slanted, then i just take a 1 quart bottle filled to the top with rotella-t and dump it in and your done, ride it some and then recheck the level. recheck it before every ride and top up as needed. i am sure by filling the bottle to the top it has a tiny bit more than a liter but its better then being low. also you can check out the "engine breather mod" if you notice oil in your airbox and oil level that keeps going low repeatedly.