[vex]

Here's the procedure for the endurance test:
A succession of mixed low-speed/low-output (2500 rev/
min and 20 hp) and high-speed/high-output (6000 rev/
min and 100 hp) sequences followed by idling, for 96 h.
Each individual sequence lasts 6 h.


Which mean's it's 19.6/96h = .204167 micrometers/hour compared to the 13.5/96h = .140625 micrometers/hour, which is significantly less than the two stroke. Redoing your math we have:
1500/.204167=7346.94 hours assuming 60 miles/hour of operation speed gives us a little over 440,816 miles.
1500/.140625=10666.767 hours of operation assuming 60 mph of operation gives us a little over 640,006 miles

Using standard oil instead of two stroke has the theoretical ability to last 1/3 longer than two stroke.