depends on your legislation:

MTBE
ETBE
Ethanol
Tert.-Butanol
iso-octane
alkylates

those are the ones i know of right now.
hth,
chris

Lead Tetraethyl (TEL) and Manganese... (MMT)are still allowed in some jurisdictions. Those mentioned by phex are usually thought of as components or extenders. Additives are usually added in small quantities like a few ccs per gallon.

HAZOP at www.curryhydrocarbons.ca

The web is full of sites on "octane enhancing additives".
I wish you a successful search. Good luck!

are there non-metallo-organic substances that would fall into the category "additives" as described by owg?
second question is: is it economically sound to use metallo-organic additives if the biggest fuel consumers forbid the addition of those? even refineries in countries not having employed these laws, our customers exclusively ask for products according to "euro 2005" norm. this is, refineries in middle east and nothern africa produce according to "euro 2005". is there still a market for TEL and MMT?

Ethyl Corporation reports that TEL earnings were down for 2002. This suggests that they still sell TEL. Ethyl also reports research work showing the benefits of MMT. This suggests that MMT is still on the market.

HAZOP at www.curryhydrocarbons.ca

TEL is manufactured by Octel and sold in many countries. Ethyl receives 32% of the revenue from all sales except those in North America. A small amount of TEL is sold to Ethyl for re-sale in North America as aviation and racing fuel.
MMT is sold in 25 countries, up from 4 in 1992. Ethyl reports that MMT is 2-3 times more cost effective than the next best octane enhancer(excluding TEL).

HAZOP at www.curryhydrocarbons.ca

thx for the info owg. never knew that it is still used...

First let me ask why you need to add octane?  Much of the need to increase octane as a vehicle ages and the pistons fill with carbon or the injectors clog can be filxed with a simple treatment of a quality cleaning product like Techron.  My Grand Cherokee pinged on premium gas (90 octane here) and I bought octane enhancers with very poor results. One treatment with Techron and I was able to get better mileage out of regular gas (80 octane) and no pinging.  I've also used it to stop run-on in Suzuki (tracker) engines and eliminate the pinging.

The word 'additives' implies substances used at fractional percent of the blend. To my knowledge, there are no organic materials that are avaialbe in commercial quantities that increase the octane number of gasolines.

Organo-metallic additives such as TEL and MMT work very well, but have regulatory restrictions.

ANY new additive introduced must go through the EPA certification process to show that it does not harm  emmissions levels and the emissions control equipment.

I recommed MTBE for increasing octane.
Cheaper is ethanol but it should be pure and i advise not more than 15 percent. Iso octane is expensive to use increasing octane and have less octane rating than MTBE and ETBE. in additon the oxygen content of ETBE and MTBE advantage for prformance of engine.