The oil change used to be the classic do-it-yourself Saturday activity to save a few bucks on auto maintenance. There are many reasons, however, why this seemingly “easy” procedure is best left to pros.

If you’re an experienced do-it-yourselfer with a convenient garage setup and time on your hands, you might save a few dollars changing your vehicle’s oil. For most car owners, changing their own oil can be a very messy lesson in false economy – not to mention possible environmental consequences. Many a would-be DIY-er has been left splashed in the face with hot, dirty motor oil, experienced burned or bruised fingers and even a damaged engine.

The seemingly simple oil change can have very expensive consequences if done incorrectly. If that sounds like exaggeration, ask yourself, Do you change the filter before or after the oil? What kind of oil does your vehicle require? What viscosity index? What performance rating?

Your owner’s manual will provide this information, and, of course, your dealer service tech knows, too. Most newer vehicles require less frequent oil changes than their older counterparts, the result of improved engines and improved lubricants. The actual change interval can depend on your driving conditions and driving style, and many vehicles even have engine oil life monitors that alert you when to change the oil.

Maybe you remember your father or an uncle changing oil and using “10W-40.” Did you know that putting that grade into some newer models could lead to damage down the road? How would you know? There’s a lot of information right on the top of a can or carton of oil in the API (American Petroleum Institute) symbol – or what’s called “the donut.” It’s a circle with various letters and numbers that tell you if the oil is right for your vehicle.

Look first for the “SAE” mark (Society of Automotive Engineers). Then look to the middle of the donut for the viscosity (thickness) information, which relates to oil flow at different temperatures. Let’s use, for example, the common 5W-30 grade, which, like all passenger car motor oils, is a “multi-grade.” The “5W” is the winter viscosity rating – the lower the number here, the thinner the oil at low temperature and the happier your engine will be in cold weather when it's time to turn over. The number to the right relates to higher temperatures, which indicates thicker viscosity for better performance in summer heat.

Now, see the two letters at the top of the donut? The first signifies engine type. If you have a gasoline engine, you need oil with an “S.” A “C” means it’s for diesel engines. The second letter represents the performance rating. For the most protection from power-robbing deposits, pay close attention to that second letter. “SM” is the best rating for gasoline engines right now; “CJ-4” is the best for diesel. These ratings change as oils are improved.

That performance rating is very important! Your new car might require SM-rated oil, so using some older “SJ” that your brother-in-law found for a bargain price might not give your engine the best protection possible. Using newer, higher-performance rated oil in older cars, however, is fine.

Finally, at the bottom of the donut, look for the “Energy Conserving” label. That means the oil has been formulated to help save gas through reduced engine friction.

Now, you can impress friends with your newfound knowledge about motor oil. Here’s another number that may impress them: The used oil from one oil change, if disposed of improperly, can contaminate one million gallons of drinking water. That’s a year’s supply for 50 people. So tell them that if they do choose to change their own oil, take the used motor oil to a recycling center. Used motor oil is routinely recycled into new lubricating oil.

Unfortunately, some DIY-ers still dump their motor oil on the ground or down storm drains or put it in bottles and dump it in the garbage. Used motor oil is insoluble, lasting and can contain toxic chemicals and heavy metals. It’s slow to degrade, and it sticks to everything from beach sand to bird feathers.

It turns out oil changes aren’t so simple.