When changing fuel filter, do you reuse the diesel drained?

If the filter isn't full of water, pour it back into the fuel tank.
If there's water in the filter, just pour the fuel, not the water, back into the tank.
It helps to carry a syphoning hose for filling the new filters.

In the old days, we used transmission fluid in the fuel, the belief being that it would clean carbon deposits from the injectors and piston rings that built up from long periods of idling in cold weather.
During winter, a quart in each tank once or twice a month was the practice, I believe type F was recommended.

 

I use transmission fluid on mine every PM but then I have a 1997 N-14

 

These cats all have valid remarks. I used to put a gallon of atf in each tank once a month but man that's expensive, so now I just put atf in the filters when changing .... and Lucas injection cleaner after the pm service. My C15 has over a million miles with no major internal work done whatsoever.