
Originally Posted by
oj88
Only if the engine was designed for it. There's a balance to be achieved. If the oil is too thick, it adds drag and loses efficiency. If it's too thin, there's lesser drag but excessive wear could become an issue.
Then again, you have to define what "old" means. To me, it can mean two things or a combination of both.... that the engine is at least 1 or more generations old and it came out during a time where standards were relatively less stringent, etc. Then it could also mean it's been worn down.
A seldom-used 'old' engine would still have close to factory-tolerances while the same or even slightly newer engine that's been run through its paces wouldn't. To a varying degree, both these engines would consume/burn oil if a thinner motor oil is used.
During the designing, the engineers have determined what oil viscosity their engine requires. If it's 40-weight oil, using a 20-weight oil or thinner will likely increase wear and tear.
Another consideration is vehicle purpose and duty. For light city driving in a modern econobox, 0W20 oils are fine, unless otherwise specified or you do a lot of spirited driving or you occasionally take it to the track. For performance and heavy duty vehicles, 0W20 would likely be too thin.