Those are both "multi-viscosity" oils. It's been so long since I had to think about this that I've forgotten the physics, but it doesn't mean the oil changes its viscosity at different temperatures. (Viscosity is measured at a specific standardized temperature). It means it will act like a certain oil when it gets really cold. A straight-weight oil will get much thicker when it's cold. A multi-weight oil will thicken less as it gets colder. That helps it flow better when starting a cold engine. It will also thin less when the engine is hot. That helps it maintain its lubricating properties. To do you a better job than that at explaining it I'd have to dig out a textbook.
Both of those oils seem awfully high for newer engines. 10W-40 used to be the most common for summer use. Some manufacturers went to 10W-30 back in the '80s and '90s. The thinner oil pumped easier so the oil pump theoretically put less load on the engine. Many engines today call for 5W-20 or 5W-30. To me that seems like water but the thinking is the oil flows easier, the engine doesn't work as hard to pump it, and fuel mileage should improve. Hmm.
Friday, September 14th, 2012 AT 6:57 AM