As I type this I am drinking a bottle of water. I need water (fluids) to stay hydrated and work my best. All people need this. Cars are like us humans because they need to maintain the proper fluid levels.
The most obvious fluid you should have changed regularly is your engine oil. That is the lifeblood of your car. Others fluids to change include transmission fluid, brake fluid, power steering fluid, differential fluid, and antifreeze/coolant. These other fluids don’t need changed nearly as often as your oil but it is a good idea to have your mechanic check them regularly.
