is it necessary to run only premium fuel, or can regular unleaded be used?

Asked by Mar 23, 2012 at 06:56 AM about the 1999 Cadillac Eldorado

Question type: Maintenance & Repair

Considering purchasing a 99 Eldorado and am getting conflicting reports on what kind of fuel to run.

14 Answers


premium fuel will give you the maximum horsepower and economy- if you don't use premium it will still run, but you won't get the most out of it, and if it knocks, go back to the premium- DON'T USE E85 (ETHANOL).

2 people found this helpful.

Some engines require premium to work properly. Check the manual or there could be a label inside the fuel trap. Modern engines have (knock) sensors that adjust to any type of fuel, so even though premium may be recomended, it will still run without problem on regular. As for performance and fuel economy, the extra cost of premium usually does not compensate for any savings in fuel economy (but! it has less emissions). Pperformance wise the difference is barely noticeable most of the time. You can also call a Cadillac dealer and ask.. its free.

7 people found this helpful.

high output engines like the north star V-8 are happyest with premium fuel .....period !

6 people found this helpful.

My husband once put regular in my 99 eldorado. You can really tell the difference. It doesn't run smoothly. The engine in that car is so smooth and powerful, it deserves the best. I use the highest octane I can get and she purrs.

1 people found this helpful.

I should have given you a better answer the first time. I have an 99 Eldorado and my husband has an 04 Seville. Both have almost the same 32 valve Northstar V-8. The eldorado takes premium and the seville takes regular. The ride in the seville is nice but it can't touch the performance of the eldorado. He tried the regular in the eldorado once because he thought same engine, same gas would be fine. The engine did not knock, but it didn't feel the same. I just use premium, a high performance engine is just that. You're talking about 20 - 40 cents a gallon. Enjoy the engine, you won't be sorry.

2 people found this helpful.



Gasoline has the same burn rate whether it is 87 octane or 93 octane. The compression ratio of the engine will determine what type of fuel you should use which should be in the driver handbook for you automobile. Higher compression engines require a more stable fuel (Higher Octane) to prevent the fuel igniting uncontrolled. If your engine has a lower compression ratio and only requires 87 octane fuel, then you're wasting money on the premium fuel. You will not get more power out of your engine this way. However, If your engine requires 93 octane to prevent Knock (Which is ignition of the fuel before the spark does it) then this cost you power. The knock sensors retard the timing to allow for this fuel but at a loss of power meaning your mpg's will suffer because you have to push harder on the gas pedal to maintain the same speeds you would at the higher HP.

3 people found this helpful.

I should have made one more thing clear on the premium fuel. I said that engines with lower compression ratios requiring only 87 octane fuel wouldn't benefit from using premium. That isn't entirely true. Some premiums have additives that help clean your carbon buildup, etc. Those additives do help with those types of things but only for cleaning purposes, not more powerful fuel.

What?! You still have 87 and 93 octane fuel in the USA? It's strange... I live in europe, and here we only have 95 and higher! 95 is the lowest octane gasoline you can get here. It's pretty much the standard everywhere in europe. Wow!


I switched to regular for a few months - no knocking - BUT I SWITCHED BACK to 93 octane because the car wasn't performing as well as I had come to expect. 1994 deville 4.9 runs better and better mileage with 93 and I'd switch to 95 if I could get it

2 people found this helpful.

Many cars that say Premium only (such as most German cars) will run just fine on Mid-Grade which is 89 Octane. In many cases however, 87 octane will not run well in those cars. Regardless of Grade - try to stick with Tier 1 gasoline because it has more additives than the minimum required by EPA.

4 people found this helpful.

I just purchased a 1997 eldorado 57,400mi. Do I have to run premium

3 people found this helpful.

Cadoo, to get the full horsepower, yes, if that doesn't matter to you, then no.

1 people found this helpful.

Thanks Richard just saw post

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