High & Low Octane Gas in Same Car?


I’m about to buy a used car (a 2001 Jetta) and I think the manual says its ok to use 87 octane. However, the previous owner has always used a higher octane gas. Someone told me if I start using lower octane gas it will harm the car. Is that true?

User "system" is causing issues

[b]Some of what you were told is true.

When a higher octane gasoline is used in an engine over a long period of time that doesn’t require it, it can result in carbon deposits forming on the top of the pistons. This is because a higher octane fuel is harder to ignite. And when it’s used in low compression engine, it can result in incomplete combustion, and the carbon forms.

When this happens, it raises the compression ratio of the engine. So now you try to switch back to a lower octane gasoline. But because of the higher compression ratio because of the carbon deposits, the engine pings under acceleration. So you end up with an engine that started out not requiring a higher octane fuel, but now requires it because of using a higher octane fuel.

So switch back to the lower octane fuel, and see how it runs. If you notice a reduction in performance because of the knock sensor, or the engine pings under acceleration, then the engine may need a combustion chamber decarbonization to get it to run on the lower octane fuel.




When a higher octane gasoline is used in an engine over a long period of time that doesn’t require it, it can result in carbon deposits forming on the top of the pistons.
Can you give a url to back that statement up? I never heard of that before.




Sorry, but this sounds like a fairy tale to me…Paying for octane you don’t need is a waste of money but I believe the two grades of fuel burn virtually identically once the spark plug ignites the fuel/air mixture. As they age, ALL engines slowly build up deposits, mostly from the engine oil, not the fuel…At the moment of ignition, there is nothing but compressed gas, no liquid, in the combustion chamber. The combustion characteristics of that fuel/air mixture is identical regardless of the octane of the gasoline that vaporized into that gas…I don’t believe the octane of gasoline alters it’s combustion characteristics at all, but that’s just my opinion…

I bet the Jetta will run fine on regular fuel no matter what grade the former owner used…

I agree with Tester.  The OP's car may not suffer from this and it does depend on the specifics of the car and the way it is driven etc. but it is quite possible.  

The advice Tester offered is solid, even if you don't buy into the theory. 

Always check the owner’s manual. It it says use high octane and nothing more use the high octane or risk damage. If it says use regular, use regular, using high test will not give you more power and could cause damage. If it says you can use either, then using regular will result in a little less power and mileage. You may or may not save any moeny.

In an older engine that calls for regular, you may need to use premium due to carbon build up, especially if you have been using premium. The carbon buil up increases compression and the need for permium.

Using premium in an engine designed for regular WILL NOT give more power or mileage. Premium is not more powerful fuel it just does not burn as easily.


“Premium is not more powerful fuel it just does not burn as easily.”

Here is where we disagree…It BURNS identically. It resists detonation (self-ignition) better. But once a SPARK PLUG ignites the fuel charge, they BURN at the same rate and with near identical BTU release. It’s the BTU’s that move the car, not octane…


Just my 2cents. run regular and if it pings or knocks up it to midgrade and see if it does the same thing if not just run mid grade. I have a policy that try and see what works. I have driven plenty of cars that state run premium and I have put in reg with no difference in performance. I believe that some cars do require premium (ex. a car with 10.5:1 compression or higher) anything else should run reg no prob and if there is a prob just up one step @ a time. there is only 1-2 octane differences between mid grade to premium.


When they first came out with “unleaded premium” and listed it as 92 octane, I thought it was a joke…That’s only 5 octane points higher than regular and was / is hardly worth the difference let alone the price.

Once upon a time, Premium Gasoline was 100 to 102 octane and Regular was 89-90 octane. Amoco sold clear-as-water lead free premium posted at 100 octane for MANY years during the 50’s and 60’s…So todays 92 octane “premium” is more of a up-selling, shelf-space, high-profit deal and offers little in consumer benefit…I see now they have opened the “spread” up to .25 cents a gallon…What a rip-off…


Thanks for the help, guys! If I buy this car maybe I’ll just have the engine decarbonized or whatever you called it to be safe. But this is all good info. Much appreciated -