I run what the book says in most of my newer cars. On older high mileage cars, I have often bumped up a "tad" on viscosity numbers assuming some wear or if I was only driving the car in the summer months. All that said, I have never had an engine failure due to lubrication and I typically drive all of my cars well past 200K even past 300k on a BMW 325. I typically just buy Walmart or O'reillys branded
cheap synthetic oil for my modern cars and same branded dyno oil for my older stuff.
Oil gets changed around when the owners manual says to change it... Never sooner but sometimes a bit later.
Here is my take on it....
New cars are built with much tighter tolerances in regard to pressurized plain bearings. They get away with this because they are manufactured with tighter tolerances and the engine temps are better regulated which means the bearing gaps stay more uniform during operation. This is why the thin oil works in todays motors. Too thick of oil will mean some of these bearings may not get pressurized enough to provide the proper oil film. Older cars like from the 70's and 80's had larger bearing clearances and thus required the heavier weight oil. Not so much anymore.