Yup, the nice folks at the ubi forum (you are fine folks as well

) told me to look in the mission editor. Outbound tankers have as cargo just freight whereas inbound ones carry fuel. That explains my suspicion.
I don't expect a tanker to go down with just one fish, even if it's hit in the engine or fuel room. What really ticks me off is that these blasted ships keep going on. At least a speed reduction (followed by isolation from convoy) would be in place.
So, very soon there will be no Ohios in my game