Oh no, another oil thread
If you want to go cheaper, get Shell Rottla T 15w40. Like $17 at Wal-Mart. Lots of big motor guys are using it, they just change it a bit more often than the pricier motorcycle oils. The builder I got my motor from (almost stock motor) used it on that motor, so I continued and haven't had a problem.
Ain't got no Ole in it
From what I've heard, the problem with car oil is because our clutches bathe in the oil. The friction modifiers/etc will KILL your clutch.
I could be wrong. It's happened once, might even happen again..
I use Amsoil synthetic 10w40 with a K&N filter and change them every 7000 - 7500 miles. I run a lot of interstate miles due to my commute and this method has been working well for me. I can get the Amsoil locally too. Use a good quality synthetic motorcycle oil and space out the changes if you are comfortable with it and your riding style suits it.
Wow, that's a big interval. How many miles on the engine?