I thought I read somewhere (on SS?) that it's not so much the miles, as it is the HOURS on the engine. I thought someone said over 1,000 hours was probably looking at a rebuild?? I would hope that one could get substantially more than that out it.
Just an excercise in math: 100,000 highway miles, at 50 mph = 2,000 hours on the clock. Assuming that you are running 2300 RPM to make 50 mph, that should equal 2300 hours on the tachometer (if my math is correct, and assuming the tachometer uses 2000 RPM as the time/rate standard). So... 500,000 miles would be around 11,500 hours! Hmmm...