Can linseed oil be used with diesel fuel to cut the cost of running a diesel vehicle?

Answer 1

Not a good Idea.
The fuel companies are already giving us diesel owners enough grief.
The government emission regulations have forced the companies to lower the lubricity & viscosity of the fuel causing pump failures, therefore I would leave things as is.
Newer fuel delivery systems are very expensive to repair.
If you are noticing a rapid increase in fuel consumption, make sure you air intake system is not obstructed. Next step is to have the vehicle plugged into a computer to check the settings.

Answer 2

No! Not unless you have a semi-adiabatic type of engine, modified for this type of fuel.