You should follow the manufacturer's recommendation for oil viscosity (as listed in all owner's manuals) based on the range of ambient temperatures the car will see until the next oil change. It's that simple, and the low end of the 20W-50 range is usually no lower than 20F, which would not be suitable for most climates in the interior US during the winter. My '88 MBZ owners manual lists 32F as the lowest for 20W-50.

On a cold start the car should be driven away as soon as the idle stabilizes, which is usually a matter of seconds, but you should use low revs and light throttle until the temperature comes up to near normal. (Note: Some MBZ models are designed to hold lower gears longer when cold to speed warmup. I'm not sure if I like this philosophy, but that's what they did.) The worst thing you can do is get right on a freeway and have to accelerate to speed with a cold engine. If you live next to a freeway ramp, use surface streets to drive down to the next on-ramp so the engine has a chance to warm up. Low to moderate speed driving also allows the transmission and axle to warm up along with the engine.

Modern engines are designed to warm up quickly, and low to moderate speed driving will usually get them up close to operating temp in under five minutes in mild to warm weather. Since most engine wear over the life of a typical automobile engine occurs during cold start and warmup, it's best to get the engine warmed up quickly, and this is best accomplished by driving the car at low to moderate speed immediately after a cold start.

The other thing to remember is that the oil takes longer to come up to operating temp than the coolant, so I avoid WOT high rev acceleration until at least five minutes after the car comes up to full coolant temperature.

Duke