Funny the seller would tell you that since Amsoil on their recommendation charts recommends for Normal and Severe Up to 2 X OEM recommendation for conventional petroleum oil, not to exceed 15,000 or 1 year.
http://www.amsoil.com/lit/g1490.pdf
Also per Amsoil, extended oil drain recommendation are excluded for modification and alteration. Examples include the use of performance computer chips, non-OEM approved exhaust, fuel or air induction systems, etc.
As per my O& Manual for a 2005 Mastiff, the recommended change interval is change engine oil and filter every 2500 miles, if one is to use Amsoil extended drain recommendation, that would be every 5000 miles, assuming no modification.
All said from my experience, oil is cheap and is the life blood of the engine. Spending $30-40 buck every 2500 miles is cheap and good insurance compared to trying to save a few dollars and increasing the chances of major engine damage due to such.
Also oil itself basically does not fail, but the additives, contaminations and abuse play major part into how good it is. Though Amsoil provide better start-up protection as well as high heater resistance to failure, like many other good synthetics do, there is still some contamination occuring from wear as well as inflitration from outside element being it mositure drawn in and of micron sized dirt.
Changing such at regular intervals will help remove such and is just a small price to increase the chance for many many happy miles and a more full wallet!
Your bike, your call. For me I use Mobil 1 and change at 2500 range, oil and filter and the primary and tranny on average ever other oil change.