In extremely cold weather, some diesel engine operators will mix small
amounts of gasoline with the diesel fuel in order to make the fuel more
combustible in a diesel engine. In that case, it's the gasoline that is
helping the diesel burn, not a case of more easily ignited diesel
assisting the more reluctant gasoline to burn as a result of
compression.

thats done to keep the diesel from turning into jello at low
temperatures. the cars i had could take up to 30% gas for that in the
winter.

Matt