Desulfurization recipes for fuel cleaner requires at least 16L of sulfur contaminated oils to perform a single run.
But distillation tower produces sulfur contaminated oils in units of multiples of 5L.
So it's hard to build the fuel producing system with only 1 fuel cleaner without importing mechanism from other mod since it's possible that the hydrogen in input tank is depleted.
This issue can be fixed by adding an extra input hydrogen tank to the fuel cleaner. Like the fertilizer solt of autofarms in Forestry.

What do you mean by adding an extra slot? I suppose you mean making a slot that only allows hydrogen to enter? Am I correct?

What do you mean by adding an extra slot? I suppose you mean making a slot that only allows hydrogen to enter? Am I correct?

Exactly.