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This may be a very noob question to ask. But I recently got this car and was used with 92 octane petrol. However, to my negligence I overlooked the user manual which states to include 95 octane petrol.

My question is, will it cause a problem if I fill the tank with 95 octane while it is quarter full with 92 octane petrol?

I did my on-line research and returned empty handed.

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    What model year is the car? If it is older than about 1990, then it might not compensate for the lower-octane fuel by retarding the spark. Thus, I personally would siphon the tank. Anything newer should be fine, just don't accelerate quickly or pull a trailer with the 92 in the tank.
    – dotancohen
    Commented Sep 2, 2014 at 11:40
  • @dotancohen hiii I am very sorry for this late reply.. to answer your question it is a 2003 Mitsubishi Galant 4G63 2.0l engine. thank you for your advice sir +1
    – Hasitha Shan
    Commented Sep 9, 2014 at 2:35
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    That Mistubishi should have no problem compensating for the lower octane fuel. Just don't step on it!
    – dotancohen
    Commented Sep 9, 2014 at 5:48
  • @dotancohen thank you sir for your reply :) sure will avoid that :P
    – Hasitha Shan
    Commented Sep 9, 2014 at 6:03

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To answer your question directly, adding the 95 to the 92 in your tank will not cause any real problems, in the short term. Once you have used the tank full you can carry on filling up with the recommended 95. Modern engine ECU programming will adjust the timing of your engine, knock sensors, to minimise or prevent engine 'pinking' caused by the lower octane fuel. Pinking can and will cause engine damage, and can be identified by a rattling from the engine, especially under acceleration.

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  • Thank you very much for the reply. when you mean 'in the short term', do you mean adding 95 octane will cause problems in the long run? or do you mean that I might have to experience issues here anad there till the engine gets used to the new fuel octane? :)
    – Hasitha Shan
    Commented Sep 2, 2014 at 9:39
  • Adding the 95 will not cause any problem as that is the fuel that the vehicle is designed for. The problem is the presence of the 92. The 92 will cause the pinking. By short term I mean, say, as an emergency use when the correct fuel is not available.
    – Allan Osborne
    Commented Sep 2, 2014 at 9:51
  • Thank you sir for the advice. there is a slight pinking going on..i will change asap :)
    – Hasitha Shan
    Commented Sep 2, 2014 at 9:54
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Mixing 95 octane gasoline with 92 is perfectly fine. In fact, this is what many gas stations do in order to offer their mid-grade product. For example, many gas stations in the United States offer 87, 89, and 91 octane gasoline. The 89 octane gasoline is a 50-50 mix of 87 and 91 octane gasoline. Mixing grades of gasoline will not harm your engine as long as the mix calculates overall to satisfy the manufacturer's minimum octane specifications.

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