Has anyone ever had a problem with a car (or heard of a direct friend) that didn't get driven much that had the wrong season's gas in it? I've heard that in extreme cases it can cause starting problems, but perhaps the Boston formulas aren't that extreme?
I've never heard of this, actually. Gas has seasonal variations? But my car isn't driven much, and has probably been in this situation on the seasonal borders at least once, and I've never had a problem with it starting.
From wikipedia: In cold climates, too little volatility results in cars failing to start. In hot climates, excessive volatility results in what is known as "vapour lock" where combustion fails to occur, because the liquid fuel has changed to a gaseous fuel in the fuel lines.
And to answer the original question, maskin77 says that the winter gas in summer vapor lock issue does occur, but he hasn't heard of it happening with a modern car.
I have heard of putting different oil in during winter, using something less dense for winter like WD30 instead of WD40. The place I get my regular maintenance uses WD30 year-round, though.
I hadn't heard of it for gasoline though. If Jered's right about it being a different percentage of oxygenate then I would expect the first symptom would be knocking. Most engines have variable timing anyways so that wouldn't really happen, you would just lose a little power and fuel efficiency.
Winter gas in summer may emit excess emissions and cause your car to fail to comply with regulations, but it shouldn't be that much harder to start on a hot day. Vapour lock is now a rare thing with modern fuel delivery systems, fuel injectors, and pollution scavengers.
Summer gas in winter isn't likely to be a problem unless the day is very (painfully) cold.
You are more likely to have your washer fluid turn to slush on a day like that.
The big concern is how long the gas is in the tank.
Older gas loses its light ends and makes starting harder.
Cat: it would be in the tank about twice as long as it now, which is maybe... a month or two.
- Not too much of a problem. Maybe on a really cold day you will have a hard start, but I don't think you should consider it an issue.
Though it did take a bit more cranking, I have managed a cold start on gas that was over a year old. On the other hand, I will need to replace the fuel in the De Lorean before attempting to start it even on a warm day.
And of course you already know not to get high octane (which is more stable) unless your car specs require it or the engine is knocking.
If you are worried about summer gas in winter, there is one more option. After the heat of summer is past, you can fill up at Sunoco on the sub-regular grade they offer.