Supertech 15w-40. Buy a gallon jug for ~$10 and have 5 oil changes on hand.
Convenient since as said, you need to change the oil frequently.
It is an air cooled 4 stroke motor that holds around 2/3 qt of oil.
Recommended oil is 10w-40, but the chart says you can use 10w-30, 10w-40, 20w-40 or even 20w-50.
There is no oil filter, just a oil screen. It is serviceable, but the engine and crankcase cover need to be removed to do it (but really is not that big of a deal). Not something to do every oil change, but if you just got it, I would do that. Here is a .pdf copy of the service manual
, it will give you tons of info on how to do stuff.
If you need a headlight, buy a new (2001+) housing, it has a replaceable bulb vs your current have to replace entire headlight housing with included bulb.
I had a friend that had one of these in 2005-2006 time frame. We trashed on that thing and it just kept going.
Fun little thing to put around on.
I have debated getting one for my daughter to ride to school (about 2 miles away, mostly through neighborhoods). Momma said no to that though...