Oil is the lifeblood of your car. The cleaner it is the better it protects crucial components. But it needs to be changed regularly to do its job properly.
Oil does not "wear out" and can be recycled again and again.
What does wear out are the additives that keep
engine parts clean and free of sludge, carbon deposits and so on.
The additives eventually run out of ability to hold contaminants and dirt particles in suspension, away from delicate moving parts. That means increased friction and accelerated wear and tear.
As to the type of oil you use, these days it's more important than ever to find out exactly what service rating and viscosity your engine needs.
Fuel economy and emission output can be affected by using oil that is too thick. And not using the grade specified by the manufacturer can void your warranty and see you footing the bill for expensive repairs.
