What causes fuel to degrade? The most obvious - evaporation - is limited to virtually nothing in any modern car, with fuel injectors that seal off when not in use and closed evaporative recovery systems.
In a carbureted engine or anything with a non-sealed fuel system, the volatile components will evaporate causing progressive deterioration of the fuel, right down to nasty varnish - but in my experience it takes a long time.
But back to modern cars. Unless anybody knows otherwise, I'm assuming that fuel degradation is virtually nothing. Other than evaporation, I can only think of three mechanisms by which deterioration might happen:
1) Settling or separation of chemical components of the fuel. My guess is this doesn't happen in any significant way in "normal" amounts of time.
2) Contamination, such as by water. Again, in a closed system, the only water that could cause this would be the small amount in the air that was admitted the last time the system was opened. In other words, after a year there would be no more opportunity for contamination than after a week.
3) Chemical reaction. Take a sealed, closed container of anything, and nothing can get in, and nothing can get out - but what's in there can rearrange itself chemically. I'd take a wild guess that in gasoline this might mean that more-volatile components would react with less-volatile components, and that the tendency would be to form components of middling volatility. Or perhaps the tendency would be towards generally higher volatility, or generally lower volatility. I'd guess that it would happen rather slowly, and that even after years and years it would be negligible in terms of actual practical effect.
Obviously I'm not an organic chemist. I'd be interested to hear a knowledgeable (or not) opinion on this. I've used Stabil (it's actually kind of expensive IMO) in lawn equipment and occasionally in motorcycles, but never added it to a car, thanks to my speculative opinion above.
- Glenn
