well first thing to do is figure out what type of bike you have and determine if it 2 or 4 stole then find out from a dealer what type or brand and wieght of oil they recomend for your particular bike. i know mine takes 20/ 50 weight engine oil.
If you look in the manual it should in there along with other maintenance information. It also may say it on the side by the oil filler. Also if you can afford it get synthetic oil.
The most important difference between oil is ignition compression (for diesel) and spark ignition for gasoline engines. After that, it's mostly personal preference. Refer to owners manual for specifics.
If you don't know what kind of motorbike oil to use, consult your owner's manual. Alternatively, you could very likely Google it and find out that way with no trouble at all.
