We usually don't think about it, but the engine in your car really takes a beating. Hundreds of times a minute a highly volatile mixture is compressed and ignited inside it.

Upon ignition the rapidly expanding gases force the pistons down in their cylinders, producing the power than makes your car go. The clearance between the pistons and their cylinder walls is measured in thousandths of an inch. Indeed, many of the clearances between the moving parts inside an engine are so small that it takes special instruments to measure them accurately.

Without the lubrication provided by oil, the engine life would be minutes instead of years: The pistons would rub directly against cylinder walls, rod bearings would touch directly against crankshaft journals, and destruction would be swift.

It's important, then, not only to have oil in your engine, but to have enough of it, and to make sure it's clean. Ignoring your engine's oil needs is an easy way to shorten its life.

Oil level should be checked every time you fill up your gas tank. With the engine not runing, pull out the dipstick and wipe the oil from it. Stick the dipstick back in its tube. Make sure you push it all the way down, or you'll get an erroneously low reading. The car should be parked on a level surface, too.

Pull the dipstick back out again and check it. The oil level should be between the "add oil" and "full" marks on the dipstick. If it is at the "add oil" mark, or below it, add; if it's exactly at the "add oil" mark, adding one quart of oil will bring the level back to the "full" mark.

It's not necessary to add oil as long as the oil level is between the "add oil" and "full" marks, but if you feel more comfortable, knowing that the oil level is always at the "full" mark, you can "top off" the oil level. Do this by adding a little oil at a time, then rechecking the level.

You don't want to overfill an engine. Excessive amounts of oil in an engine can cause problems.

What type of oil should you use? The one recommended in the owner's manual -- it won't specify a brand name, but it will tell' you what type, such as 10W-30 (commonly used).

Should you use a synthetic? That's up to you. Tests of good synthetics are very encouraging, but synthetics are much more expensive per quart. And they should be used only in new or low-mileage cars. Switching to a synthetic in a high-mileage car can cause more harm than good.

To add oil, remove the oil-fill cap or plug and pour the oil in. The oil-fill is usually on the valve cover. There are several ways of adding oil.

You can use a Phillis screwdriver to punch two holes in the can across from each other (one a pouring hole, the other for air), stick a funnel in the oil-fill hole and pour the oil in. Of course, you can use an old can opener to make the holes, too.

Or you can use one of the inexpensive commercial pouring devices. There's the spout, such as found in filling stations. You just push it in the top of the can and pour the oil out. There's the plastic funnel that clamps on top of the oil can (you punch the pour holes in the top of the can first). It has a cap for the spout so you can use it as a cover for a partially used can of oil. And there's the trigger-operated oil fill, sold as an accessory; but while it works well for pouring the oil in, it's sometimes hard to remove the can from the device after you've finished.

All three of these devices are available in some auto-parts stores and stores that have auto-parts departments. The plastic funnel that fits over the top of the can is also available in some motorcycle shops.

Probably the simplest way for the do-it-yourselfer, though, is just a cheap plastic funnel and a can opener.

Change your oil regularly, too, and put a new oil filter in at each oil change.