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This is usually one of those 'hot-button' topics - along with what brand of brake pads or what brand of shocks to use - so I'm a little surprised that there hasn't been a lot more discussion. Anyway, here goes...
20W50 is on the heavy side unless you are pulling an Airstream Classic across the Nevada desert in summer. 15W40 is the max I would use, but normally it would be 10W30/10W40 for spring/summer/fall, 5W30 when the winter is in full swing (Canada, eh.).
Dino oil SAE rated at 10W40 will flow the same hot or cold as an SAE 10W40 synth, but the real difference IMO is the higher temperature capability of the synth and the lower tendancy for coking. Regardless, I still change the synth every 5000km, because, well, dirty oil is still dirty oil.
I wouldn't say the heavier oil necessarily reduces wear; in fact, the very opposite may actually be true, as the oil takes longer to reach every part, and may be difficult to reach into the smallest of areas (such as piston skirts) on a well-maintained or low-mileage 'tight' engine.
The heavier oil will cause extra drag on the engine in general and also put extra strain on the oil pump and pump drive in particular. I had a timing belt jump on a car (not a Volvo) when I drove from the warm west coast to a winter resort, due mainly because of a loose belt (i.e., neglect) but made worse by the 20W50 I was still using at the time and the extra strain it put on the intermediate shaft (which drove the oil pump and distributor).
What the heavier oil can do is help reduce oil consumption (rings/valve guides) and piston blow-by on high-mileage, loose engines.
But in the end, no matter what shop you choose, if you don't like their default 20W50 viscosity, they should put in whatever you ask for - but you do have to ask.
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