It appears I missed a few questions

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Is the centre a movable space?
Yes. The board is that shape so that most factories regardless of the country in which they reside, are equa-distant from each other.
In a 3 player game, are the hospital and R&D zones in the 4th player area dead?
Yes. Any country that is unoccupied is counted as but 1 space for movement for the entire country, and it is considered desert for game purposes; nothing exists there, and the entire 1/4 of the map is counted as only one space for movement.
What happens to oil when the carrying unit is defeated (destroyed or given to attacker)?
The oil is captured and may be used by the victorious attacker, but ALL opposing units must be destroyed for this to occur.
Can you move a unit with oil to a square with units and on the same movement phase move units out of that square using up that oil?
Yes. In our at-home games this is known as "leap-frogging".
When attacking a square with multiple units and one unit was carrying oil, can you specifically attack the unit carrying the oil, or is it just kept by the owning player unless all of their units in the square are destroyed?
"B", it just kept by the owning player unless all of their units in the square are destroyed.
If there is no defense or labour units in a factory, do you still get 5 labour units for conquering it when you move in?
Yes. Don't forget that if there are Labor Units there, however, you'll need one attacking unit PER Labor Unit in order to destroy them all and take the factory. Fodder, anyone?
Thus if you take one, and move out, could someone else conquer it simply by moving in and get 5 labour units,
Yes.
...or must you defeat a garrison to get the 5 labour points?
No.
Would a factory revert to it's original ownership if you remove your garrison in theirs after taking it?
No. The factory remains the property of the last player that conquered it until another player conquers it.
Hint: If your selling tanks, don't take an opponents tank factory unless you know he has the means to retake within a turn or two - you'll cut your own market share severely. By the same token, if your opponent sells planes, food, oil, etc. you can severely cut his ability to market by taking an opponents factory of that type ... hehe..
XXOOCC
The guy that runs this chicken outfit.