So, what is it like driving a diesel that uses waste fryer oil as fuel instead of diesel?:

You get into your modified Diesel car, truck, or motor home and start the engine. Since there is diesel (or biodiesel), in the main fuel tank, your diesel engine starts up as it normally would. You flip the Grease4Fuel toggle switch activating the thermoswitch for the vegetable oil fuel system and contemplate your 30 minute drive. In the few minutes it takes to drive out of your neighborhood, hot engine coolant begins circulating in the heat exchanger of your Grease4Fuel conversion tank.
As you drive for approximately 6 miles, the vegetable oil in the conversion tank is heated which increases it's viscosity into warm, flowing liquid oil, suitable for fuel injection.

When the thermoswitch senses that the vegetable oil fuel system has reached operating temperature it "automatically" switches to turn the solenoid valve on. The valve from your normal fuel tank "closes" as the valve for your vegetable oil fuel conversion tank "opens".
Your Diesel engine does not miss a beat.

Enjoying the spring time drive you roll down the window to let in some fresh air and notice a subtle sweet smell of french fries. You may also notice that your engine idles a little more quietly due to increased lubricity. About 1 mile before reaching your destination, you flip the toggle switch to the Purge position to change the engine fuel source back to the main diesel tank. As you park, your engine has replaced the vegetable oil from the fuel pump and the injectors with the bio-diesel (or diesel), from the main fuel tank.

You turn your vehicle off and wonder why nobody believed you when you told them you were going to drive your car for mere Pennies using waste veggie oil as fuel!