Oil lubricates the moving parts of your engine. It keeps them cool
by lowering friction and carrying heat away. It also keeps any grit or
tiny particles from scratching tightly-fitted parts. Heat breaks down
oil. It can make the oil gummy or watery (or both), so that it doesn’t
lubricate properly. And the longer it’s in the engine, the dirtier it
gets. Replacing old oil with new fixes all these problems. The fresh oil
lubricates and cools better. When you flush the old oil out, you also
flush away any grit or dirt in it. Getting your oil changed regularly is
probably the single best thing you can do for your engine. To reduce
repairs and improve performance, follow your manufacturer’s oil change
recommendations listed in your owner’s manual.