Choosing an Appropriate Oil Weight

Times Staff Writer

Question: The owner's manual on my 1985 Buick Century is somewhat vague. For oil changes in my former cars, I aways used 30-weight oil. Do you recommend 30 or 10W-30 oil? Is it OK to mix oils, since I have several quarts of 30 left?--E.K.

Answer: Proper oil weight is an issue that generates a lot of disagreement among the experts, but everybody would agree that you should never mix different weights of oil and especially never mix multiweight and straight-weight oils. The oils often will have incompatible additives that could cause engine problems.

Deciding whether to use a straight-weight or multiweight oil is somewhat more complicated. If you live in an area where the temperature does not drop below 40 degrees or rise much above 100 degrees, you could safely use a straight-weight oil.

Some experts believe that straight-weight oils have better lubricating properties. Multiweight oils are based on a light-weight oil, such as grade 10. They contain so-called "viscosity improvers" to make the oil act like a 30 or 40 weight oil at higher temperatures. Over time, these viscosity improvers can break down, resulting in thinner-than-desirable oil.

Still, the trend among auto manufacturers is to require thinner and thinner multiweight oils for new cars. In 1986, for example, 5W-30 is not uncommon. The reason is that manufacturers can pick up a fractional improvement in their corporate fuel economy ratings and ease low-temperature operation of small engines.

Q: I have a 1984 Mercury Marquis. I am worried about the clicking sounds of metal cooling and contracting after I park the car. Is there a danger of fire?--J.J.

A: Your car is most likely acting normally when it cools off. In fact, new cars click from the sounds of contracting metal much more than older cars because of catalytic converters.

Catalytic converters, which are used to burn off emissions in the exhaust system, operate at temperatures in excess of 1,000 degrees. When the hot exhaust stops flowing through, they cool down very quickly and make a lot of noise.

Q: I own a 1975 Ford LTD with a 400-cubic-inch engine. My problem has to do with oil pressure. The engine light comes on at times when I stop at a signal and then goes off when I go. The auto has 90,000 miles and I use 10W-40 oil. Should I use a heavier oil?--S.V.

A: A heavier-weight oil is not the immediate solution to your problem. If the engine light is operating properly, then the engine is not receiving adequate oil pressure.

First, you should check to see if your car is idling at the proper speed. It should idle at 650 revolutions per minute in neutral and 625 rpms in drive. The oil pump is driven by the engine, and too slow an idle speed will not permit the pump to keep up the required pressure.

If the idle speed checks out, then you should have your mechanic check the oil pressure with a manual gauge. He will unscrew the engine sensor for the dummy light on your dashboard and attach a pressure guage to the engine that will tell whether the sensor is operating properly.

The engine should be getting at least 15 pounds per square inch of pressure at idle speed. If it isn't, then you could have major engine problems, such as a worn-out engine or a deficient oil pump.

The lubricating system is designed to pump oil through a series of passages in the engine. A worn-out part along the circuit will allow the oil to escape before it lubricates subsequent parts on its path through the engine.

While only a costly overhaul will fix the problem, using a thicker oil or an engine additive that thickens oil will help somewhat.

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