Yes
Depending on where the leak is, if there is a weakness in the 350 it was with the oil cooler lines. It turns out that most of the oil leaks from the cooler lines has nothing to do with seals. It's a problem with the cooler line.
Dozens....
Not without some serious mods to the carb, fuel lines, seals, gaskets, etc. E85 will eat through rubber in no time and will really dry stuff out.
Rubber gaskets are used as seals between two objects, like a lid to a jar. Usually, rubber gaskets are used to prevent something from leaking. http://en.wikipedia.org/wiki/Gasket
The seals go on first, then the tabs on the ends of the gaskets lay in the channels molded into the seals. A little dab of RTV sealant on each corner helps to avoid leaks.
Jaguars have plug seals.
The seals and gaskets on your gas tank are aging OR you have a crack or microfissure in your filler gas filler neck OR gasket and lines for overflow gas reservoir are cracked and aging.
The effectiveness of automotive leak inhibitors vary dependent to the level of damage/degradation of the engine, transmission, cooling, and steering systems seals and gaskets. Engine oil stop leaks seal to recondition seals. Severely degraded seals and gaskets usually require replacement.
replace valve cover gaskets & spark plug tube seals
There could be a number of reasons. Gaskets could have failed. (Head gasket, valve cover gasket, or oil pan gasket, for example.) Seals could have failed as well. it's also possible there's an obstruction in the PCV system, which could have pressurized the crankcase and blown out seals. If your vehicle has an oil cooler, the leak could be there. Could be damage to the oil pan itself.
Bad valve seals will that. You can purchase oil treatment which allows partially restore failing gaskets. But if the engine is doing that for a while, you need to replace the seals.
Internal oil consumption. External oil leakage from seals and gaskets.