Lower octane is a bad idea...you stand a better chance of detonating under load, which can lead to a blown head gasket. I REALLY doubt it would ever happen, but why chance it. I run 93 from Chevron and NOTHING else. I also try to run some high rpms once a month to keep the engine clean.

Also, doesn't the manual say Premium fuel only? With higher compression motors you need to run high octane.