Quote Originally Posted by PaulMillner View Post
.... However, Petersen Aviation has an STC for those Lycoming 91 octane engines to run on premium unleaded ethanol-free mogas. I don't recall the minimum octane they specify, but I think it's 93 AKI, pump octane, which brings us back to my original comment... it's difficult to find that stuff ethanol free. It's nearly impossible to find that stuff in the West....
When in doubt, look it up. don't need 93AKI for Petersen STCs:


http://www.autofuelstc.com/stc_specs.phtml
http://www.autofuelstc.com/

but cost savings? around here we can get 100ll avgas at the airport for as low as $4.87, octane-unspecified-on-the-web mogas at the airport for $4.90, or ethanol-free 90(?) octane boat gas at the local stop&rob for $5.00. name yer poison, pilgrim. your mileage may vary.