I'm not sure what's going on here to be honest, but my first guess might be that you don't have your FHs set to be tankers in the class design window, if I don't have that checked I can't tell my ship to unload fuel, and it may be possible that a non-tanker can't unload fuel under any circumstance including default orders.
Edit: just tested, if I try to do that conditional order with a non-tanker ship I get an error message telling me to check my conditionals, and no fuel is unloaded.
Yeah that did it. THANKS A BUNCH