How long does unleaded petrol keep for?

Deterioration of petrol is mostly caused by oxidation. The rate of oxidation will depend on temperature (faster at higher temperatures). Storage in the boot of the car over the summer months would almost certainly result in deterioration. Badly oxidized petrol acquires a darker color and a “skunky” aroma. It also becomes cloudy, and may have visible particles of gums, which are the products of oxidation. These are what cause engine damage.

Rather than have to remember to rotate your reserve, you might want to consider buying a bit of petrol stabilizer to the reserve. This is an anti-oxident that will greatly slow the deterioration. It can last up to a couple of years with a stabilizer.

You also should keep in mind the RVP, Reid Vapor Pressure of the fuel. High RVP fuel will evaporate quickly and aid starting in cold weather. Low RVP fuel is best for summer driving to prevent excess vaporization, and spit back wen you open your filler neck. Depending upon where you live and the expected temperature extremes the RVP is adjusted as many as 8 times per year. In the winter, it will be very difficult to start a car that has run out of gas if you only have low RVP fuel (Summer fuel).
Fuel stabilizers like Stabil say they are good for a year but I’ve kept it longer and mixed it in my tank. My record was 5 years. I found a 2 gallon can in my barn and just put it in my tank when it was more than half full – no problem .
1999 Buick LeSabre with the 3800. It eats anything.

I left a car with a full tank parked for a year and when I started driving again it was just fine.

One year is perfect timing. It’s the people who filled up with summer fuel, let it sit for 6 months and then try to start the car in the dead of winter (0F) who have trouble. A shot of ether Starter spray will usually kick it over.

I’ve always marked my cans of fuel with the type and date and will use any that remains after a year has transpired.