My daily driver El Camino came with a v8 305 from the factory. It went 300,000 miles and that engine was worn out, and started smoking.
I took that engine apart just to see what was worn and how clean it was. The valve guides were shot, and the piston rings were worn. The bottom end was very clean.
I took it to the scales and sold it as scrap for $70 bucks. I installed a brand new 350 that was $1,700 delivered to my door. That was way cheaper than any machine work on an old engine.
Just regular oil changes and oil filters kept it clean. Then and now, when I change the oil, and check the level on the dip stick I have to get a reflection off of the dip stick to see the oil on the dip stick as it is clear. It only has a little color after 3,000 miles.
My 911 is the same way. Very clean inside.