Changing oil is the simplest action, however, many beginners have problems with it because of poorly made tutorial. First, you must open the quick menu (RMB) when you stand in front of a car and select the icon on the bottom. You will move a car on the car lift. Then, you must press the button on the side of the car lift twice to get a car high up in the air.
Now, go to the oil drain and from the menu you can select where to move it (under a car). When the device is under a car you must click on the oil drain again and select use option. The oil will be drained in a few seconds. After that you must take the device and lower a car.
Warning - if you, for example, twist off an oil pan the oil will also be drained. However, you will have to pay (100$) for cleaning the oil from the floor.
When you open a hood (by clicking on it) you must hold LMB on oil fill plug and a bottle of oil will appear. You must click again and hold the button to tip the bottle and pour oil. You must do it until the pan's angle changes by itself (optionally, you can pour a little bit more). The process is now done. The oil fill plug goes back automatically. You can check oil level by holding LMB on oil bagnet.