Oil & Filter changes are an easy part of Car Mechanic Simulator. They are frequent in the early game. Some oil filters may be a bit more difficult to identify and find, but once you know where they are it's an easy job. Of course knowing where the oil drain is and how to line it up does help.
You'll be doing a lot of oil and filter changes, especially in the early game. In general the oil filter is located on the under side of the engine. You'll find the drain plug somewhere on the oil pan, which is attached to the bottom of the engine. Keep in mind that the color of the oil filter is different depending on the type of engine. Also keep in mind that it's a totally different part and location for the Maluch.
The oil drain itself is located near a ladder close to the main garage doors. Once you have it, approach the car from the front center, looking up as far as you can. When the little dot lines up with the oil plug, quickly press ESC to release the oil drain. Then left click to enter disassemble mode. Now all you have to do is take out the oil drain plug.
Don't forget to change the filter and to move the oil drain out from under the car. Of course it's also a good idea to remember to put oil back in the engine. If you miss with the oil drain and it hits the floor it will cost $100 to clean up the mess. If you are doing this for a repair job it seems as though the client pays for your mistake, but if you are rebuilding a vehicle you purchased at auction, then it's on you to pay for it.
To fill the engine with oil, simply take the oil cap off. This is located on one of the head covers. All you have to do then is left click and hold it to pour in the oil. When it is filled correctly, the oil can will tip back up. You'll need to release the mouse and then press ESC to put the oil can away. If you overfill the engine, don't worry. It doesn't let you fill it past the max line.