Synthetic In High Mile Engine?

In the wifes car, since it doesn't leak, I run sythn oil. Only a 130,000 miles.

Crude in the Magnum since it does burn it up a bit. Unknown miles.

Crude in the Duster for now. This engine came from the Magnum. I have documents with over 300,000 miles when the orginal stopped writing it down in 1989 when he gave the car to his Daughter for 4 years of collage in upstate N.Y. (I live down state.) and then to his son for the same reason, 4 years as well. He then used it for a daily driver to work 50 miles away for 5 years when I got it. I just flipped the odmeter again.
I only changed out cam bearings with the new cam. Otherwise, no major repairs.

If it dosen't leak after some time, I'll go to synth.