It was in the owners manual because fuel gelling was more pervasive at the time and this was the remedy for extreme weather conditions. For example living in Artic conditions. Mercedes-Benz vehicles are sold all over the world, and the manual wasn't written just for the people in North America.

That doesn't mean that it was a good idea, it simply means that it was a stopgap solution to a problem. Just like using unleaded gasoline is bad for a Coleman stove, but if you are hungry.....edit: coleman dual fuel stove, not the older models,sorry I should have been more clear.

It was a bad idea then and now because gasoline in any percentage mixed with diesel is going to damage the injector pump due to it's lack of lubricity. Gasoline, even in lower concentrations can cause detonation in the combustion chamber, similar to the effects of using ether in a diesel not designed for it. Mechanical damage can include bent connecting rods, destroyed rod bearings, and piston breakage. Power output will be reduced also.

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"I was a dirty bird, Carol's not grungey - she's *****in" John Milner....American Graffiti