Q I bought my Volkswagen Jetta new in 1999 and there had been no excessive oil use until recently. Now the oil just seems to disappear—it’s down more than a quart at 3,000 miles. What’s wrong?

MARIANNE YURKONIS   
   Marina, California  

A Cars tend to lose oil as the miles mount. There are only two ways for oil to get out on its own: Either it leaks (often messily, but sometimes with an almost malevolent ability to conceal the deed) or, as wear increases, oil can get by piston rings and join gasoline in the combustion chambers, where both are burned. Unless you see a small puddle of the stuff on your garage floor, using just over a quart in 3,000 miles isn’t cause for worry.

Q On a recent trip I noticed that in Idaho and parts of Nevada, regular gas is 85 octane rather than the usual 87. Would an engine perform the same with 85 as with 87?

ARTHUR DOI  
Sacramento, California  

A The car’s performance might be marginally affected. The octane number indicates a gasoline’s ability to resist premature burning in the engine’s combustion chambers. The higher the number, the greater the resistance. Most cars are designed to use regular gas and, as a rule, will not run more efficiently or produce more power if you use gasoline with an octane rating higher than the one recommended for them.

When the fuel and air mix burns before it should, stress inside the engine increases, vibrations result, and the engine may register its pain and displeasure by making the rattlelike sound of knocking. Usually, regular gas is 87 octane, midrange is 89, and premium is 91. Although most cars use regular, engines that operate at higher internal pressure need higher octane fuel. Atmospheric pressure decreases as altitude increases and this lower ambient pressure effectively lowers pressure in engines, too. So cars at high altitudes can, at least in theory, run on lower octane gas. In addition, today’s computerized cars can to some degree adapt to lower octane fuel. If your car objects to 85 octane gasoline by knocking, try the next higher grade.

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