Premium Gas and Regular Gas


I have a 1998 Mazda Millina with the Miller engine. The manual says Premium gas. I looked at the discussion of Premium gas when I entered the web site. My question, which was not answered in the article, is: Can I alternate a tank or regular with a tank of premium? Can I use mid-range instead of Premium? What do you all think? Thanks, Robert


There are differing points of view on this issue and my answer is - it depends.
State of tune, type of car, atmohpheric conditions, whether the knock sensors are operative, if the EGR system(very critical on this issue)is working, etc.

Some vehicles take lower octane fuel with no problems; others may be cantankerous.
91 is recommended for my Lincoln Mark and I’ve been using 87 in it for about 150k miles. At altitude (Colorado) I’ve used 85 with no problems at all; no loss of mileage or performance at all. It still gets 27 MPG on 85 octane even while in the mountains.

I’m also very picky about the state of tune and remove the spark plugs about every 10-15k miles for a tip inspection just to see how things are going. Zero problems at this point.

If the plugs show normal, there is no pinging or loss of performance, etc. then I don’t have a problem with it at all.

You might do a net search for how octane rating is figured and read up on it. The 87 may not be as bad as you think.
Others will tell you that 87 is instant death; I happen to disagree.


Does the manual recommend premium or does it say it needs premium.

If it recommends then you will likely loose some power and mileage. Judge for yourself from the results if you want to continue.

If the manual says use premium then not using it can cause damage to some engines. If yours is one of those don’t try the mix thing. The engine can be damaged each time it gets regular.


Not worth the hassle of remembering which one was the last grade put in the tank. Use the premium


I used to own a 2000 Milennia S, with the Miller cycle supercharged engine. My experience was that there was a significant loss of performance with both mid-grade and regular vs. premium. While it probably won’t destroy the engine to use lower grades, I didn’t find it cost effective due to the reduced mileage and power. Also, I have heard it said that it really doesn’t make much sense to use the lower grades with a turbo or supercharged engine because there is a greater potential for exceeding the capabilities of the knock detector.


“The manual says premium gas.” If that’s what it says, don’t use anything else. EVER. The engine in your car is supercharged. It NEEDS the octane in premium to operate correctly. It’s my opinion that the engine will suffer internal damage over the long term if you don’t use the correct fuel.

If you don’t want to buy premium gasoline you really should purchase a vehicle designed to run on regular. A Mazda Millenia is not such a vehicle.


A couple of points here; feel free to answer.
Allegedly, 87 is not good enough for a car that requires 91.
The same gasoline that is rated 87 in the U.S. could be shipped to Europe and will be rated as 91, at a minimum, or better there. Why would it be “good enough” there and not “good enough” in the U.S?
Why would shipping gas to the U.S. from Europe lower the octane rating and vice-versa; why would it be acceptable to run a “no 87 car” in Europe on the same gas?

Has anyone in their lifetime ever seen any engine damage at all that has ever been caused by using a lower octane fuel (racing/performance vehicles excluded)?
Has anyone ever seen any spark plug damage (the first victim) caused by low octane fuel?

I’ve been turning wrenches for about 35 years and I have never seen, nor heard of, one example of octane caused engine damage. While exchanging war stories with mechanic friends they state they have never seen this either.
I HAVE seen engine damage caused by air leaks, improper ignition timing, improper valve adjustments, spark plugs, etc., but nothing that was ever gasoline caused.

(Also, I’m not condoning the use of a lower octane fuel if the engine is pinging at all; only stating that most of the time someone who is suffering pinging may very well have another problem (EGR?) causing it.)

Points for discussion anyway and just curious about responses to those points. :slight_smile:


The point is, why would you want to risk it? Maybe the lower octane fuel will cause damage, and maybe it won’t. I’d be willing to bet that it would happen if regular was persistently used over a long time, particularly where the manual says that premium is required. Will you destroy the engine from one fill-up? No, probably not. But repeated fill-ups over months or even years? I could see it happening.

The octane difference between continents, I have no idea about. I can’t understand why 87 here would be 91 over there…is it not a universal rating?

In any case, I just do not understand why anyone would shell out the big bucks for an expensive import car that requires premium fuel (as stated by the manufacturer, who really knows), and then start hedging over whether it really needs it. Is it worth saving a few cents per gallon to even risk engine damage? If they can afford the car, they can afford the fuel. Nobody can stop them from using something else, but it’s at their own risk.


What type of vehicles do you turn wrench for? The OP’s car has a supercharged engine. I’ve seen supercharged engines get destroyed because of pre-ignition, even though these engines were set-up for racing. I’ve seen holes punched into pistons and heads cracked.

Modern cars have a knock sensor designed to back off the ignition timing to prevent pinging ang knocking, but, as a previous poster has mentioned, there is some discussion on whether or not the factory timing adjustment limits are enough for supercharged engines.

I certainly would not risk it for saving $3 a tankful (20 cent extra for premium in a 15 Gal tank).


As I mentioned in the earlier post, I’m leaving racing engines out of this discussion. That’s a whole different deal and we’re talking street cars here.
I have to question the sanity of anyone who destroys an engine due to pre-ignition. It’s not like the damage occurs instantly so this means that someone was really whaling on it and ignoring the severe rattling that had to precede the damage.

As to what I’ve worked on; everything from Renault LeCars to GMC/Ford 2-ton bobtail trucks and just about everything in between. Both forced induction (turbos and supercharged) and naturally aspirated.

Matter of fact, I own a supercharged SBF Ford right now with shaved heads, cam, no knock sensors or EGR system, and I can beat that car into the pavement on 87 octane with not a peep out of it.

Still curious about the same gasoline having a 4/5 point octane rating difference.
Is 20 gallons of 91 octane gasoline shipped overnight FEDEX to the U.S. not acceptable for the Mazda? The gas will be 87 once it hits the border.


There ARE different octane ratings for the same gas. If I recall correctly, they are the “motor rating” and the “research rating”. In the US, there are 2 ratings which differ by several points. The applicable rating in the US is the average between the 2 ratings. You don’t need to do the arithmetic…the rating on the pump and in the owner’s manual is the averaged rating. Probably, the non US pumps use the higher of the 2 numbers.