Question: What are the advantages of using synthetic oil, and how often should the oil filter be changed when using synthetic oil?--M.W.
Answer: The key advantage to synthetic oil comes in very cold climates, such as in North Dakota or Minnesota, because synthetic oil has very good flow properties in low temperatures.
Synthetic oils are excellent lubricants, but they may not be a bargain. In some instances, such as in cold temperatures, they can offer performance that no other oil can match. But in routine driving outside the cold-weather regions of the country--where temperatures drop below zero for much of the winter--you may not need a synthetic oil.
Some synthetic-oil producers suggest that these lubricants do not need to be changed more often than every 25,000 miles. Some experts question this advice, including Norm Hudecki, Valvoline's associate director for research.
Valvoline itself is introducing a synthetic oil, so Hudecki's advice is not a criticism of other oil companies. He claims that the additives in synthetic oil are the same as organic oil and subject to the same deterioration. So the same logic would apply to the oil filter, because up to one quart of dirty oil remains inside the filter if you don't change it.
Many experts, including Hudecki, recommend oil changes every 3,000 miles, regardless of automobile manufacturer's recommen- dations for two or even three times that interval. Oil is one of the cheapest maintenance items for your car.
Q: I have a 1971 Mercedes 280 SE, which has a six-cylinder engine. I have always used premium leaded gas until a little over a year ago when I started using unleaded gas. I notice now that my car doesn't have nearly as much power and the engine does not idle smoothly. Did I make a mistake in using unleaded gas?--F.W.S.
A: You should have your engine checked to make sure that you did not cause premature wear of the valves and valve seats inside your engine. Leaded gasoline lubricates and cushions engine valves.
Newer engines can operate on unleaded fuel, because the valve seats are made of hardened metal. With the earlier valve seats, the lead inside fuel created a lubricant and a cushion that would prevent wear.
Once the valves or valve seats are worn out, you can lose engine compression, which results in a loss of power and roughness.
You can't be certain that your problems are caused by premature valve wear. You need to have a compression and leak-down test performed by a good engine mechanic. It's quite possible that other problems are causing your difficulties.
You should be using leaded gasoline. You might consider mixing unleaded premium with leaded regular. Or you might also consider using one of the many fuel additives available to simulate the role of lead in your fuel.