I used to hang around a mechanics garage in the 60's. It was unusual to see them working on cars with over 80,000 miles. 100k mile vehicle was an oddity and quite the novel thing that someone put so many miles on. Most people didn't drive over about 5,000 miles a year back then. Many a lot less. When they did work on a high mileage vehicle and pulled the valve covers off it was sludged up really bad. Oils back then were not as good and engines were not as tight and carburetors didn't atomize gasoline nearly as well so the oil had a bigger job to do. 2,000 mile oil changes were common. Some people changed at 1,000.

I remember my dad buying a used (which was unusual for him) Plymouth Belvedere, think it was a 66, (one of the ugliest cars ever made) with a slant six and push button automatic. It was a few years old at the time and it only had a couple thousand miles on it (school teacher of one room school house owned it - one room schoolhouse closed down and she retired - she didn't drive but her husband did and he was getting to old to drive).

That 225 slant 6 automatic would do an even 100 mph top speed. Was a good car other than being ugly. Drove it to high school some when I could find an excuse to not ride the bus.

Those old engines would make around 100k miles or a little more before being worn out. But nothing like some of the modern ones. 100k miles back then was like 300k on a good car now. Of course the newer ones get lots more miles per year which helps. Many families 10-20k a year if they drive a ways to a job.