I live in South Carolina and currently run Suzuki 10w30 in my Eiger. Would a synthetic such as Amsoil or Mobil1 do a better job of standing up to the heat of my summer (90+)?

 

Here in the southwest desert, we know about heat! I run 10W-40 in winter, and 20W-50 in summer. Personally, I am partial to Honda HP4 (a pertolium/synth blend) in the sport bikes, and Honda GN4 for the autoclutch bikes. Don't know what kind of clutch your Eiger has, but some synth oils are not compatible with an autoclutch.

Good article if you haven't seen it yet:
http://www.thumperfaq.com/oil.htm