there's a big difference between a car engine and a motorcycle engine. probably the most important difference is the clutch. in a car, the clutch is typically a dry type, or doesn't share the same sump with the engine. on a motorcycle (spcificly this one) the clutch IS wet, and uses the same oil to lubricate it's parts that the engine does. because of this, the oil MUST have certain properties that do not allow the clutch plates to slip. It has been shown that, generally speaking, oils that are labled as "Energy Conserving" contain friction modifiers (or Moly) not suited to a wet clutch system. because of that, you must use an oil that is either certified JASO-MA or one that contains trace amounts of Moly to no moly at all. typically, all multi weight w30 grade oils all are "Energy Conserving" and are thus unsuited for operation in a wet clutch motorcycle. That said, you can never go wrong using a motorcycle specific oil. if you dabble around the auto parts store you will likly find other 10w40 oils that are not energy conserving. understand that while they may not say "Energy Conserving" on the API label, they very well may contain too much moly for the clutch. symptoms of a unsuited oil will be clutch slippage under load (while riding and/or accelerating).
There are many many manufactures that produce oils suitable to use in a motorcycle. Under regular change intervals, one oil will be no better than the other, as long as you change it. conventional oils degrade quickly, whereas synthetic oils last considerably longer. Amsoil is a great example. I use it in my bike and after half a season of riding after the break-in change it's still good (as per my oil analysis test concluded).
bottom line is, if you change your oil religously, one oil will be no better than another. Use whatever makes you smile. if them means using the cheapest conventional on the shelf... fine, just change it out when it's used up. there's no easy way to tell if the oil is used up though. the only way to know for sure is to get it tested. some oils don't hold up to the shearing forces that happen inside the transmission as well as others. so while, say, a Castrol will hold up well for 2000 miles that does NOT mean a penzoil will, or a QS, or M1, or a host of other lubricants. they might be similar, but they are not the same.
so choose wisely...