I figured this was all snake oil
So, I talked to a friend and...
OK, first who he is.
He is what is referred to as "Subject Matter Expert" by the US Navy.
You know, the people that use the largest, heaviest, most amp-hour wet-cell batteries there are (ie they are in subs, yes even nuke boats have propulsion batteries).
His statement, yup, they are snake-oil.
Now the Navy really isn't concerned about cost effective solutions, when lives and mission are on the line, the best solution is what they want.
Yes the oil might reduce water evaporation, except that when the vehicle is in motion the oil is sloshed up on the cell and the water and oil would constantly be mixed. It's the oil being splashed on the exposed electrodes that is another real concern, this oil coats the electrode, impairing the functioning of those electrodes.
Final thought, if you want to extend the life of your batteries, do proper maintenance.