While I don't run used oil in my 7.3, I do run a 25% mixture in my 6-71 Detroit.(the old 6-71 really doesn't like this new fuel) When I have clean hydraulic fluid, I will run as much as 50% mixture of it. I have run this engine for several years at these rates with no adverse affects.

I have a friend who is running an older Mecedes 4cylinder non turbo on 75% used oil and 25% gasoline. He has run this car for over 2 years like this, and has no problems with it. He does have a mixing system which filters down to 2 microns.