Originally posted by Steve Gutman
Why not run straight 30 in the summer and straight 40 in the winter?
Some people DO run straight 30W... if you're guaranteed the temp will be above 50° when you start the car.
The reason for multiviscosity is to provide decent lubrication to the engine during startup, otherwise straight 30W (or 40W)would be best for the engine.
But like I said, it's a trade-off. We run 10W30 (or whatever your preference is) because we need the 10W behavior during startup and the 30W behavior for operating conditions.