Synthetic engine oils can cause oil leaks?

Back in the 1970s, when synthetic engine Oils (those based not on petroleum but on chemical base stocks such as polyalphaolefins) first became popular, they didn't always play well with the seals and gaskets in the car's engine. They could cause the seals to shrink in ways that petroleum-based oils did not, resulting in those messy oil leaks that would mysteriously appear in your car's parking space. Some people still fear that synthetic oil will cause leaks and so they continue to use petroleum-based oils instead.

These fears are largely unfounded. Oil manufacturers long ago learned to reformulate synthetic oil so that seal shrinkage doesn't occur. Still, there's a way in which synthetic oil can cause a leak, at least when you use it in an older car that's been operating for years on a petroleum-based oil. The synthetic oil can clean oil sludge off the seals that may actually have been blocking off tiny cracks in the seals, revealing leaks that have been there all along. This probably won't be a problem on newer cars, but if you're still driving a car that's more than, say, 15 years old, you might not want to make a sudden decision to switch to a synthetic oil.

Should you change your oil every 5,000 km, no matter what the manual says?

Once upon a time, almost every auto manufacturer recommended that the oil in your engine be changed every 5,000 km. Use oil past that interval and the engine would begin to fill with sludge, which would not only degrade performance but leave the moving parts at risk for damage.

That's no longer true. Modern detergent oils, improved oil viscosities and better auto engineering in general now allow cars to go about 12,000 km between oil changes. Yet you'll still hear the 5,000 km figure quoted widely -- especially by people trying to sell you oil. No less an authority than Consumer Reports has debunked this myth, stating that unless you drive your car under unusually difficult conditions, and especially if you always drive it in stop-and-go traffic, going 12,000 km between oil changes shouldn't harm your engine in any way.

When Engine Oil turns Dark, it's dirty and should be changed?

If you're conscientious about keeping your car in good running order, you probably worry from time to time that your oil has gotten dirty and is causing sludge to build up in your engine . So you pull the dipstick out and check the color of the oil at the tip. Chances are, it's starting to turn dark, no longer the light amber color that you saw on the stick when your oil was fresh. So now it's too dirty to use, right? It's depositing sludge in your engine and needs to be changed.

Wrong. In fact, just the opposite is true. If you're using a detergent engine oil (and most modern engine oils have detergent additives), the oil is working just the way it's supposed to, dispersing the tiny particles that can result in engine sludge and holding them in suspension in the oil itself so that they can't build up. That's why the oil appears darker, but this in no way impedes the oil from performing its normal functions of lubricating and protecting the metal surfaces inside the engine. Of course, the oil is limited in how many of these suspended particles it can contain and will eventually need to be changed when it becomes saturated, but use the oil change interval recommended by your car's manufacturer to decide when to change the oil, not the color of the oil on the stick.