When arriving at the Palomino Creek Garage, walk up to the door (with the blue house icon).
When walking up to the door, you will notice something pop up on your screen, telling you the business information and to use /enter to enter into the building.
Once purchasing your engine oil, head towards the door and use the command /exit to leave the building.
Once you are outside, head towards the car and enter it, then use the command /bonnet.
After you open your bonnet, use the command /changeoil and your character will get out of the car.
When you are out of the vehicle, head towards the marker that'll be located in front of your vehicle, and use the command /changeoil again.
If you are unsure what the marker looks like, it should look like this.
(If the marker doesn't show up for you, push your car backwards a bit as the marker might be in the front of the car for you)
Once you have arrived at the marker, use the command /changeoil and there you have it- you have successfully changed your vehicles engine oil.