Could the OM617 be the most reliable automobile engine ever made? A very good case could be made for it, although its successors haven't been around long enough for us to judge whether they might be even better. Sure, diesel engines have the advantage of using a fuel that is itself a lubricant, and they generally operate at lower RPM ranges than gasoline engines... but once you get past 500,000 miles (as so many 70s and 80s Mercedes-Benzes powered by the OM617 have done), that line of argument loses much of its power. Make the jump to see some video of this engine in action. [Wikipedia]
Here's some video shot of a vegetable-oil-powered Mercedes-Benz at the 24 Hours of LeMons race last October; another veggie-oil diesel Benz finished 19th at that race.