Dohc
DOHC heads are different beasts - with 32 valves, the odds that one is cracked. loose (a "floater") or simply leaking are increased just from the standpoint of having twice as many of everything. As for a sporadic problem, the positions of the 32 valves is rarely the same when you shut it down - 4 cams are making sure of that. The leaking valve being open at shut down to allow the pooling of oil would therefore be happening about half the time, at most. Combine this with the fact that the seal may only leak some of the time. If it's loose and squirming around, it may often seal at rest, even though it doubtless leaks other times and assuredly while the motor is operating.
About the only way to be sure of the solution is to remove the heads - which the dealer obviously doesn't want to do. Next trip to the dealer, be firm that they MUST fix the problem, or you'll have no choice but to complain to the regional Ford rep.
Has the dealer run a compression test on the motor? That would at least tell us if the bottom end is as it should be. If it turns out to be a vacuum-related issue, fine, but they seem to have already tried that fix...