I was just checking the oil on my car when it reminded me of a gas station scam that has been played in the past. Now that so many gas stations are self-service, perhaps this scam is practiced less. But just for your information here is one way crooks will attempt to cheat you. It's called short sticking and is or was most common along interstate routes where drivers are covering a lot of miles in a day.
Short sticking. You're in the middle of nowhere when you stop for gas. You are delighted that this is a full-service gas station. The helpful attendant washes your windshield and checks your oil. Good thing, too, because you are a quart low. He shows you the dipstick and you can see for yourself that the oil level is at the bottom of the stick. Helpfully, the attendant sells you a quart of oil (at inflated prices--maybe twice what you'd pay anywhere else--but what can you do?) and installs it for free. He shows you the dipstick again and the oil level is right up where it should be. You drive away relieved and thankful that the problem was found and taken care of.
Rewind a minute. This scam is called short sticking because the attendant didn't put the dipstick in all the way, resulting in the appearance that the car was a quart or more low. But don't worry, the attendant didn't overfill your crankcase with superfluous oil. He sold you a can of imaginary oil, an empty can that he pretended to pour into your car. He's probably sold that can of imaginary oil to 36 people just today. And imaginary oil has a 100 percent profit margin.
How to prevent this scam. This scam is easy to prevent. (1) Check your oil each morning while on a long trip. Most modern cars don't use much, if any, oil these days, but you will feel more confident if you do check. (2) When confronted with a dipstick that shows your oil is low, say, "Really? Let's wait a minute for the oil to settle in the crankcase. On this car it can take a few minutes. And then I can check it again. No need for you to take your time. Thanks for your help." Note that this little speech precludes the attendant's comeback of "Don't you trust me?" and gives a perfectly good reason for rechecking the oil yourself.
Scam insurance. If you really want to be on top of the game, keep a quart or two of your car's oil in the trunk. If you're told you're a quart low, follow the script above, and add, "Luckily, I have some extra oil with me, so I can put it in if need be."
Useful learning. Learn these things about your car: (1) What type and grade of oil it uses (conventional, synthetic). (2) What viscosity (for example, 10W-30, 0W-20, 5W-20). (3) Where the oil filler tube is and how to take off the cap. (4) Where the dipstick is and how to read the oil level.