If my car can take E10, can it use unleaded 91?
No, alcohol has a very high octane number.
>>18123735
Are we talking AKI or RON here? Either way, you'd need a cut above regular unleaded so if we're talking RON, you'd be fine running unleaded 95.
I'm sure it can, but is it optimal? If your engine has a compression ratio of less than 8:1 then it is within specs for 91 octane. Whatever fuel you use, running your engine at a moderate load (not flooring it in high gear) and regular oil changes, does more for its longevity than using the best fuel.