Start with straight 93 unleaded (E0) and it makes best power at X degrees of advance...

Add 10% ethanol, and you'll require more ignition lead.

Instead of adding ethanol, blend in 50% leaded fuel and your ignition timing requirements will change again.

Start with 93 E10 and then blend in 50% leaded fuel... you're working with yet another set of ignition timing requirements.

This doesn't touch on the changes to the AFR that go along w/ the changes in the fuel being used, either. Stoich for straight 93 is around 14.7; 93 E10 is closer to 14.0; 110 leaded is closer to 15.0...

My apology for stating the obvious if everyone already know this stuff...