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Follow-up file
A number of readers who read last week’s column “How Often Should I Change My Oil” wrote with other oil-related questions.

I own three vehicles, all 2005 or 2006 models. All three call for 5W-20 oil. I had never heard of this before. I am seeing 5W-20 more available on store shelves now, but a few months back it seemed somewhat difficult to find. Is this a fairly new blend and what’s the reason for it/
-- Steve M, Crandon, WI

Yes, 5W-20 is fairly new oil, designed for today’s more sophisticated engines. As John Ibbotson, an automotive expert at Consumer Reports, explained to me, “because the tolerances are tighter and things are smaller, you need a thinner oil to get through the passages and to keep things moving and lubricated.” Ibbotson says this is especially important on cold starts where the oil has to rush to the engine after sitting overnight. “The thinner 5W-20 oils do that much better,” he says. They can also help your engine run better, which will improve fuel economy. Remember, older vehicles still need thicker oils. Use the grade of oil called for in your owner’s manual.

What are the advantages of using synthetic oil? And if I decide to make the switch, when should I do it?
-- Steve, Wareham, MA

The companies selling synthetic oils will tell you their products are far superior to conventional oil and worth the significantly higher price. They claim synthetics can reduce friction, decrease engine wear, prevent rust, protect against harmful deposits, deliver better performance in temperature extremes, decrease oil use, and last longer between changes. Synthetics have another advantage: they don’t come from oil pumped from the ground.

A number of high end vehicles, such as the Mercedes SLR, Cadillac XLR, Chevrolet Corvette, Bentley GT, and Aston Martin have engines designed to run on synthetic oil, so they come filled with Mobil 1.  If you have one of these vehicles, or if you're planning to race at the Indy 500, you would absolutely want to use synthetic oil. Otherwise, you need to consider cost vs. potential benefit.

Mike Calkins, Manager of AAA's Approved Auto Repair program says, “the engine in the average motorist’s vehicle is probably not operating under conditions where the added protection offered by synthetic oils come in to play." However, he says "it can be beneficial if you do a lot of towing and your engine is constantly under a heavy load." Calkins says synthetics are also a good option for people who live where it gets extremely hot in the summer or extremely cold in the winter.

The makers of synthetic oils say you can go longer between changes, in some cases as far as 15,000 miles, but both AAA and Consumer Reports recommend following the service interval listed in your owner’s manual, especially while the car in under warranty.

A number of new vehicles, including models from Honda, Mercedes, BMW and General Motors now have oil change indicator systems; a warning light goes on when the vehicle senses it’s time to change the oil. AAA's Calkins says we'll see more cars offering this feature in the future.

I've been using synthetic motor oil in my car and have been told if I switch to regular oil there is a strong possibility the seals will start to leak.  Is this true?
-- Ruth F

Synthetic oil manufacturers say their products have been designed to be compatible with all the seals and gaskets in the engine. I asked Ibbotson at Consumer Reports about that and he agrees. “Switching from synthetic to conventional motor oil or vice versa,” he says, “is not a problem. Of course, if your engine already has an oil leak, you should have that repaired before switching to a synthetic product which may make the problem worse.

Is it good to use synthetic oil after the car has more than 100,000 miles on it?
-- Greg P., Flovilla, Georgia

Not really. If you’re using a synthetic oil to reduce engine wear and extend engine life, you need to do that when the engine is new, well before it hits 100,000 miles. “If your engine has over 100,000 miles, and it is worn and possibly burning oil, synthetic oil will not plug the leaks,” says Phil Reed, the Consumer Advice Editor for Edmunds.com. “In this case, you can buy heavy weight synthetic oil, but if you’re burning oil, why not burn cheaper oil?”

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