The webpage , http://www.turborick.com/gsxr1127/gasoline.html
, has a very good explanation on gasoline's properties. In Section 6.15 It states that :
6.15 Can I mix different octane fuel grades?
Yes, however attempts to blend in your fuel tank should be carefully planned. You should not allow the tank to become empty, and then add 50% of lower octane, followed by 50% of higher octane. The fuels may not completely mix immediately, especially if there is a density difference. You may get a slug of low octane that causes severe knock. You should refill when your tank is half full. In general the octane response will be linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.
Attempts to mix leaded high octane to unleaded high octane to obtain higher octane are useless. The lead response of the unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary gasoline can result in undesirable increases in volatility due to volatile azeotropes, and that some oxygenates can have negative lead responses. Also note that the octane requirement of some engines is determined by the need to avoid run-on, not to avoid knock.