I know of a vehicle approaching 500,000 mi with 15W-40 that speced 5W-20.
I know of a vehicles that have went 600,000 - 750,000 mi on 5W-20.
The truth is most vehicles are not that sensitive to oil viscosity.
OP - The only difference you will note is in very cold weather you will get oil pressure faster with 0W-20. Otherwise they are functionally identical.
You know of a vehicle approaching 750,000 miles on 5w20? That’s a lot of miles. It must be a fleet vehicle.
I have never in all my years working in the trade of automotive, seen a vehicle with more than 300,000 miles on it. 300,000 was the absolute most - it was a Acura Legend with the V6 - it was actually in pretty condition and looked like it could have gone another 100,000 but the owners had a limit of 300,000 on all of their cars. They’d buy new and keep them all to 300,000.
I have seen a customer spend $5,000 on repairs/paint on an old Lexus LS400, that had 250,000 miles. Thought they were crazy, but they wanted to keep it in the family (sentimental). I also saw a lady put a brand new crate transmission in a Hyundai mid size four door sedan, when the car had 250,000 miles. Her logic was, the car was in great shape otherwise, the tranny repair was only $1,500 total, and it beat making car payments. And I agree with her.
I personally switch to a heavier weight oil as the vehicle starts to get higher in mileage. If my vehicle calls for 0w20 - I may run the 0w20 up until 150,000 miles and at that point I will consider switching to 5w30 (depending on if it’s using some oil). Sometimes it makes a difference, sometimes it doesn’t. I have seen many techs and people do the same.