I have read countless engine manuals over the years. Vehicles, outboards, marine diesels, reciprocating airplane engines from Lycoming, Continental, and Pratt & Whitney. Lawn mowers, chain saws, generators.
And with the exception of the two stroke engines that instruct one to mix CLEAN 2-stroke oil into the fuel, not ONE of these manuals ever said "After you have drained the crankcase of the old, contaminated lube oil please pour it into your fuel tank so your engine can run on it and you can save a couple of bucks."
Whether it kills the Lehman or not, it sure isn't going to help it. It will also clog up the fuel filters faster so one might negate the savings in fuel cost by the cost of more frequent filter changes.
Oh, he knows they do and mentioned such in our phone call. But he said that this doesn't make it a good idea. It's to accomodate people-- mostly big companies according to him-- who for whatever reason are trying everything they can to cut costs. That's one way to do it.
So your go through filters faster and you accelerate some of your maintenance costs, but the savings look great on paper and the engine manufacturers are more than happy to accomdate their customers who buy bazilions of dollars woth of engine parts and support over the years. "Want to cycle your old lube oil through your engines? Hey, have at it. It just means we'll make more money off of you down the road."
So says he, anyway, and he was in the business a long, long time.