What Does the Engine Oil Do?

Oil is a lubricant, whether you are talking about cooking oil in a pan or spray lubricant to stop a door hinge from squeaking. The oil in your car's engine keeps everything moving smoothly, and reduces friction for more power with less fuel consumption. But engine oil also cools and traps dirt, which is why it degrades and needs to be changed.

Even with modern liquid cooling car, the oil also cools the vital moving parts. Engine coolant circulates through passages in the block and head, but can't flow past the pistons, valves, crank and camshaft the way oil does as it lubricates. In a turbocharged car, oil to keep the turbo bearings cool and lubricated as well. Heat breaks down the oil, making it less effective as a lubricant, which is why it needs to be changed, even if all the contaminants are filtered out.

The moving parts of the engine rely on a thin film of oil to act as a cushion as well. Internal combustion is a violent process, with hundreds of little explosions every second to move the pistons. Changing reciprocating motion (up and down) into rotational motion (round and round) puts a lot of demands on the metal parts. Liquid oil keeps the metal parts from touching, so they don't quickly wear out. That is why it is important that oil is the correct viscosity, or thickness, and heat changes that over time.

What Does the Oil Filter Do?

Oil Filter Change Image

The engine oil also works to reduce wear by flushing abrasive particles away from critical areas. The oil filter catches tiny metal particles and contaminants that could otherwise damage engine parts, and keeps them from recirculating. Without a filter, oil changes would be every 1-2,000 miles, and dirt and metal would collect in the bottom of the oil pan.

Due to modern car design, oil filters can only be so big and can only hold a limited amount of contaminants. Go too long between filter changes, and it may become clogged, triggering an internal bypass. When this happens, any additional dirt or metal will just continue to circulate with the oil.

Every car is different, so find your manual for the full instructions…

When to change your oil and filter

“Some models need to have their oil and filter changed every 5,000 miles or five months”

This is often debated among DIY mechanics. Every vehicle and engine is different and requires a different answer - always consult your car’s owner's manual or refer to your Haynes manual for the correct answer for your car and style of driving. Some models need to have their oil and filter changed every 5,000 miles/five months (whichever comes sooner), while other can go twice that. Towing, driving in areas with a lot of dust, frequent short trips in cold weather, high ambient heat, or the strain of mountain roads, usually means you should follow the accelerated, severe condition maintenance schedule.

Most 21st-century cars have a computer reminder which uses all the data available from the OBDII systems, including ambient temperature, load, hours driven, etc., and tells you when a change is due. If you do change the oil and filter before the car reminds you, your Haynes manual will tell you how to reset it.

All cars are slightly different, so if it is time to change your oil and filter, use our before you begin checklist, and find your car for specific instructions.

How Much Oil is Needed and What Type?

Every car is different, and you will find the answer in the front of chapter one of your Haynes manual. Some small cars need three to four quarts of oil (some motorcycles take less than one!), while larger trucks and air-cooled cars can take up to ten quarts!  though. Without the manual, you can measure how much you need based on how much comes out when you drain it, then check the level on the dipstick (but remember, once is is drained, you can't drive to the auto parts store!).

Oil comes in a number of different grades and viscosities, or thicknesses. The recommended type is printed on the oil fill cap or a sticker under the hood. Modern oils have less zinc in them, which is an anti-wear additive still needed by many cars from the mid-1990s and before, so you may need a specific oil or additive for older cars. Your Haynes manual will list the original factory specification for your car.

How To Change Your Engine Oil and Filter

This video is a general guide. Find specific step by step instructions in the manual.

A very brief summary of the task:

  1. Run the engine until it is up to operating temperature. Shut it off.
  2. Raise and support the car. Place a drain pan underneath the engine.
  3. Remove splash shield if needed to access plug and filter.
  4. On engines with a paper filter element, remove it now.
  5. Undo the oil drain plug and allow the oil to drain.
  6. Wipe the plug and plug hole clean, then reinstall and tighten to specification.
  7. On engines with an oil filter canister, remove it now with a filter wrench or by hand and drain remaining oil.
  8. Fill new filter with clean oil, if applicable, and wipe some on the gasket.
  9. Install new filter and tighten it by hand only.
  10. Open the oil cap and fill the engine with fresh oil.
  11. Start engine and be sure oil light goes out, then shut it off.
  12. Wait a few minutes, then check oil level on dipstick. Top off as needed.


Never be tempted to skip a filter change - dirty oil may bypass the filter if it becomes too clogged with contaminants.

Tools you will need

Typically only basic tools are required for this job, although you’ll need a filter wrench. You will need to raise the car and you may need to remove a splash shield underneath the motor to access the drain plug and filter.

  • Drain pan
  • Rubber gloves
  • Ratchet and socket set
  • Filter wrench
  • Funnel