It depends on the percentages of the wrong fuel in the tank.
Much too much diesel in a gas engine will destroy it as it burns longer, hotter and will fry the exhaust valves and make the exhaust manifolds glow bright red. Seen it. Once running on diesel, a gasoline engine is VERY HARD to shut off as typically a gasoline engine (with a carburetor) will not stop flowing fuel, but it will also not require ignition to keep it running as internal engine parts are by then glowing hot enough to sustain ignition of the diesel fuel.
Gasoline in a diesel is just about as dangerous, but in different ways.
It will NOT lubricate the injection pump (if it has one) or the injectors or their pintles and it is thinner in viscosity and is therefor less controllable in the system. Gasoline has a much lower cetane value and it will "knock" dangerously and can damage the piston tops, rod bearings and piston pins, crack rings and ring-lands, even damaging the glow-plugs (again, if it has them) and any pre-combustion chambers that the engine may use.
I've never heard of putting gasoline into diesel to make it less susceptible to freezing. Usually, stations just sell a lower-paraffin content fuel in colder months as the paraffin wax is the agent that coagulates and falls out of solution and creates a mass of rather "sand-like" precipitate in the bottom of the fuel tank. It is unpump-able and the lift pump will run dry and the system will lose prime and cease motivating.
"White gas" is another old-wive's tale to add to the mystique of keeping diesel fuel from freezing. Don't do it either.
Just use Diesel #1 in colder weather. Diesel #2 is for use in regular temperatures.
The only "bollocks" involved here is lack of thermodynamics knowledge.
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