I was under the impression that each country pays the oil upkeep for it's own units but this isn't the case. It appears that the top unit is the determining factor as to which country pays the entire oil upkeep of the fleet.
For example if I move the French fleet to sea and then add a a UK ship, the UK will pay the entire Oil upkeep.
Another example and extra cheesy/exploity is that I can move the French Med fleet to the north sea using barely any oil by using the Canadian DD. Canada having negative Oil seems to do no harm except that Canada stops sending it's 5 oil to the UK.
To do this, simply move a country's fleet first then move another country's naval unit to the same fleet. The last nation to move a unit to the fleet will pay entire oil upkeep of the combined fleet.
