Question: I've been seeing more advertising about the benefits of synthetic motor oil, but does my car or minivan really need it? Answer: The conditions you drive in may determine whether you use synthetic oil. Synthetics help protect the engines of vehicles operating in extreme conditions such as sub-zero cold or overpowering heat.

For most cars, minivans and sport utility vehicles, conventional oil will do just fine.

"If you want to pay the extra money (for synthetic oil), we have no problems with that,'' said Bob Olree, a materials engineer responsible for motor oil in the powertrain division of General Motors Corp. "But for the typical car owner using his car in typical service, it's probably not money well spent.'' Motor oil lubricates the moving parts in an engine and allows them to keep moving with a minimum of friction. It also cools the engine.

Conventional motor oil is made from crude oil, while synthetic motor oils are manufactured by chemical reactions to produce a fluid that acts like conventional oil but leaves fewer potentially troublesome byproducts like carbon sludge.

The experts say that when deciding which to use, check your vehicle owner's manual to see what the automaker recommends. In most cases, it will be conventional motor oil. GM recommends it in all of its models except the sporty Corvette.

Premium grade motor oil typically sells for about $1.50 a quart, while synthetic oil goes for $4 a quart and more.

Despite the extra cost, synthetic oil sales at mass merchandisers like Kmart or Wal-Mart climbed 17 percent over the past year vs. a 3 percent rise for conventional motor oils, according to Julianne Oberg, product manager for the synthetic oil Castrol Syntec in Wayne, New Jersey.

But she said synthetics still account for only about 4 percent of overall motor oil sales volume at such outlets.

Don Johnson, an expert in lubricants technology at Pennzoil-Quaker State Co.

in Houston, which markets both conventional and synthetic motor oils and blends, said synthetic oil offers two big advantages. He said synthetic oil flows faster at extremely low temperatures such as may be found in winter in Minneapolis or Buffalo and helps engines start quicker. It also works better at keeping the engine cool in summertime traffic jams, he said.

Someone who makes regular 100-mile trips on the interstate and changes the oil every 3,000 to 5,000 miles may do fine with regular motor oil, Johnson said.

But if you are towing a trailer or are regularly getting caught between buses and trucks in rush-hour traffic jams in hot weather, Johnson said synthetic oil may be worth the extra money.

Using a synthetic oil in such conditions need not commit you to the higher-priced oil forever, though.

Synthetic oil Synthetic and conventional motor oil can be used interchangeably without harming the engine, Johnson said.

John Nielsen, director of the auto repair network at AAA, the automobile owner's association based in Orlando, Florida, said drivers should change motor oils regularly no matter which kind of oil they use.

But he said "a Cadillac doesn't operate much different than a Saturn in terms of engine heat.'' "For the average consumer, it feels good to use synthetic but as a practical matter, it's not going to make a major difference to the car,'' he said.