Use Dictates Need for Oil Change
Question: I have a 1991 Toyota that I drive about 300 miles per month. How often should I have the oil changed? If I wait until I have driven 3,000 or 4,000 miles, it would be once per year.--C.Y.
Answer: Under normal use, Toyota recommends that you should change the oil every 7,500 miles or every 12 months, whichever occurs first. But under severe use, it suggests changing the oil every 3,750 miles or every six months.
If you are accumulating mileage at the rate of just 300 miles per month, you are certainly within the definition of severe use. You are probably taking only short hops in the car, during which the engine barely gets hot. Under such driving conditions, the amount of water condensation in the crankcase, fuel dilution of the oil and soot contamination rises.
Normally, those contaminants are burned off or held in check by driving the car until the engine reaches normal operating temperature. But if the engine barely gets hot, the water will continue to accumulate. A cool engine also will not fully burn fuel, resulting in increased dilution of the oil. And the unburned fuel generates greater soot.
In general, oil is judged bad if it contains water condensation in excess of .3% or if it contains enough gasoline to dilute the oil by one grade in viscosity (meaning that a 10W-40 oil changes to 10W-30). As a typical motorist, you have no way of knowing when these limits are reached, so it is better to be safe. Some oil experts recommend changes even more frequent than six months under severe driving conditions.
Q: I own a 1989 Cadillac, equipped with a V-6 engine. My dealer advised I burn premium unleaded fuel, because it is supposed to contain cleaners and other additives that are better those in regular. But the owner’s manual suggests that I use regular. I’d rather not pay the extra money, but I don’t want to ruin the car. Who’s advice should I follow?--J.I.
A: Some premium gasolines do contain a better additive package than regular, but some brands of gasoline use the identical additive package in the regular and premium grades.
These additives contain anti-icers, detergents, corrosion inhibitors and agents that retard the formation of gums and varnish.
Most likely, your Cadillac has a fuel injection system that could be seriously damaged by dirty gasoline. A good additive package with detergents will help prevent clogged fuel injectors.
But many regular grades of gasoline contain good detergents. In general, gasoline retailers would never dream of telling consumers something as useful as whether their premium grade fuels contain a different additive package. But sometimes you can discern such information from the way the gasoline is advertised or promoted.
If the premium is promoted as having superior ability to keep your engine running clean, it is likely to have a superior additive package. This is admittedly imperfect.
Since many brands of gasoline contain the same good additive packages in the regular grade, you might not need to pay the extra price for premium. You are not really paying for the additive package with premium fuel. Rather, you are paying for a higher octane rating.
The octane rating is a measure of the gasoline’s ability to resist uncontrolled combustion and the resulting engine ping. If your engine does not ping, then it is unlikely you need premium fuel.
Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, Times Mirror Square, Los Angeles 90053.
