With the widening gap between the cost of regular grade gasoline and E85 gasoline -- also known as flex-fuel -- the E85 gasoline looks more and more attractive to motorists looking to curb their fuel expenses. Your 2003 Cadillac Seville, however, cannot use flex-fuel. According to General Motors Service Information bulletin No. 04-06-04-047H, your Seville benefits from the use of a gasoline formula designated as "Top Tier."
General Motors recommends the use of an unleaded gasoline of at least 87 octane in the 2003 Cadillac Seville. When used for towing or when you want increased performance, use a middle grade or premium gasoline. If 87 octane isn't available, a lower octane may be used, but diminished performance can result. Switch to a higher octane gas as soon as possible to prevent damage to the engine.
The use of fuel additives is not recommended since all gasoline sold in the U.S. already contains additives to prevent deposits from forming and is formulated for modern emission control systems. However, General Motors does advocate the use of GM Fuel System Treatment PLUS -- part No. 88861011. General Motors does not recommend using gasoline containing methyicyciopentadienyi manganese tricarbonyi. MMT is a compound that may be found in some gasoline brands sold in Canada.
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