1st we have all those "additives" that claim to improve power, and torque, improve "smoothness" emissions, and mileage! Sometimes by 25 percent! But they cannot, or will not tell you how! Well get real! They are a CON!
2nd we have "octane boosters" SOME of these work well. Look for tests and recommendations, or just use proper high octane fuels in the first place!
3rd we have "upper cylinder lubricants" such as Redex etc. Well in a road car they may not hurt. In a highly stressed race engine running on the borderline of detonation, it might just be enough to kill it! You see oils radically lower the total cylinders Octane So be careful!
4th Methanol. This can if added in small amounts increase octane. But it may make the total fuel mixture weaker so again be careful!
Then there are the lead replacement additives. These may or may not work, some definitely don't. They are designed to replace the lead that some engines like to stop valve/head seating problems in pre unleaded fuel days! Don't confuse lead and Octane. Lead is for lubrication. Octane is a measure of the fuels ant knock properties. Although lead happens to increase octane a little too!
A little known fact...
In the UK leaded pump fuel if you can get it when mixed with super unleaded gives a higher octane than either alone! Because both use different additives to increase octane that work less effectively the higher the concentration