Fact is 5w20 is not going to do any harm
Sorry dude, I have to disagree. Register at BTOG and search for "52-20 junk" and you will many posts from guys you had their motors damaged on 5w-20, some very quickly. You will find posts on engines that start to knock on the 20 stuff.
A reciprocating engine has certain minimum needs, regardless of whether it is a hard worked 4-cylinder, a loafing big block V8, a Lycoming ait cooled 6-cylinder. That minimum need is about what we call "30" on the oil bottle label.
Let's play some numbers. Let's say that 5w-20 is 99% as much protection as 5w_30. That would mean that in 40,000 miles you will have driven 400 miles with insufficient oil protection. Would you ever do that on purpose? No, of course not.
Look, the single highest cause of engine wear is starting it up. There's no oil in the system, the bearings are "dry" and it takes a couple of seconds for oil to flow through the engine. Unfortunately, we can't do much about that. I did have a race car in the 80's that I put a pre-oiler on. It was a pressurized tube (like an oversized shock absorber) with a floating piston, an air chamber and a check valve. When you shut the engine off, it trapped about a quart of oil under pressure. When you were ready to start it again, you flipped a switch, gave it 2-3 seconds to see about 10 lbs of pressure on the oil gauge, and then started the engine. Of course, that was not practical for street cars.
Other than an accumulator, the next best protection you can give your engine is to have high levels of zinc in your oil (manganese is good too), this added oil in the form of zinc phosphate. Unfortunately, a little oil is burned by even the healthiest engines, and phosphates degrade the performance of catalytic converters. In 2004, under US law, manufacturers had to start warrantying cats for 120,000 miles. So manufacturers changed their oil requirements to drop most of the zinc. You can add zinc to your engine oil, but you do risk shortening the life of the cats. (I do run a smidge of a zinc additive in my oil, but it's still under what was in good oil say back in 2000/2001.)
The second highest cause of engine wear (assuming you're NOT the type that leaves oil in a car without changing until it dies), is oil film break down at higher oil temps. In August, inching along in 100 degree heat in heavy traffic with the A/C on, you are a prime candidate for oil film breakdown. Even our "little" 2.5's stress the oil hard enough to experience the conditions that can exceed the protection of a 5w-20 oil.
It's not like 5w-30 costs more, so you have nothing to lose. OK, maybe you get 0.2 less mpg, but I'm willing to pay that price. I'm not willing to pay the price of those 400 miles of insufficient oil protection in 40,000 miles.
And like I said, it doesn't even cost more!