Gasket is bad or bolts around perimeter of pan are in need of a light tightening.
ANS 2 - A typical cause of this is people who tighten the bolts round in a circle, as opposed to the way it should be done, which is doing all up fingertight, then going side to side tightening opposing bolts, so the gasket does not get clamped on one side and loose on the other.
THATS usally cause by the engine using oil-because of bad oil rings etc.
A major leak.
Sounds like a possible oil pan gasket leak, also check around the oil filter and make sure that it seated properly when it was installed.
If the thermostat was not installed correctly, the housing can crack but it would leak engine coolant not oil.
can exhaust manifold leak cause you to burn oil
I would not expect a valve cover leak to cause a car to stall. A valve cover leak can fill the spark plug area with oil which can cause rough running.
engine oil leak
No
Changing the plugs and plug wires correctly has nothing to do with nor can it cause an oil leak.
An oil leak is always going to be out of the engine as the engine contains oil and the car does not. I think you are saying you are loosing oil with no apparent leak. If that is the case the engine is burning oil and that is why you see no leaks.
Leaking valve guides in the head will cause oil to get around the spark plugs.
they leak oil from the oil pump on the front of the engine