Fred,
There is an unsustantiated report of a very high hour Lister engine on the lister CS owners forum. You may want to check that one out. Nothing to do with osl, of course. Anecdotal reports suggest that some CS's have had very high-hour lifetimes. These would be 'out of the crate' genuine Listers probably.
That said, there must be the possibility of these engines lasting many many thousands of hours with appropriate maintenance and servicing. That would, I would think, include regular replacement and/or refurbishment of some of the moving parts.
Original unused Listers are now few and far between, as they have been out of production for over 20 years. I do not believe a Listeroid would be likely to run as long (trouble-free) as an original might have done - unless it had a lot of time spent on it. Most Listers would not run trouble-free for that length of time either.
I personally (my opinion, of course) don't believe a word of it but might be persuaded if the documented, and fully sustantiated, evidence was available. I am talking here of actual examples, not a computer program providing the 'evidence' from the inputted data.
There are always exceptions to a rule (well, often anyway) but I doubt anyone would expect engines to last that long. I think if you read the claims very carefully, you could expect it to fail and expect it to be repaired free of charge - but only if it was proven to be operated in a certain way all its operating life. Those extra costs would likely be more than purchasing more than one engine for any particular duty.
Finally what is the guarantee worth? Perhaps 20 years down the line.
In my opinion it is unsubstantiated sales hype, but I could be persuaded otherwise. I am waiting, like a lot of others to see any hard evidence that every one, or any one, engine would actually last that long any better than a good example operated in a similar ideal way.
Regards, RAB