Well the thing is, you would only get oil on the window if..
1:the top of the engine took enough hits to *blow up* and thus enough oil would get thrown in all directions.
2:If its just hole, its highly doubtful it would spurt against the wind just to it could ruin the pilots rear view.
The shaft will most likely not be a singled out damage component, i can instead however see its as being a *part* of the engines damage system. Same with the transfer case up in the nose, im willing to bet if this is the case, the p39 will be a very easy aircraft to take down, simply because if you are to strike from nose to rudder, you would hit transfer case,cannon, machine guns,pilot,engine,fuel,rudder.
Thats alot of damage options for one pass on a plane.
lets hope her armor stands stuff.
:Edit:Vans, the batt was probably in case the engine died on takeoff/landing or due to combat damage, the pilot could still operate,safely land the plane.
it would have sucked badly to take off, engine stall, and not be able to put your gear down "or flaps" because they have no power from engines.