If you checked your Engine oil
and found the level to be low, you should add a quart. Motor oil is sold in quarts, so if you grab a plastic bottle at your local giganti-gas, you've got a quart. There are different types of motor oil, called "weights," so check your car's owner's manual to see what they recommend. If you can't find the manual or you're in a pinch, you can always add a quart of 10W-30 or 10W-40 safely (they're labeled right on the front). If you're extra worried about cleanliness, buy a funnel, too, but it's not a must.