Hello! 2007 car, 1.8 duratec with 5 speed manual transmission. Bought car damaged, replaced piston rings & usual overhaul. I broke PCV valve during disassemble and as I couldn't get OEM in that hurry, I replaced it with Chinese knock off I had. Car has been losing oil, quite much. I suspect the valve got closed with high flow. That is now fixed but couple things puzzles me. 

Under battery there I a lot of oily dirt and also transmission bottom is oily. Looks like transmission oil has increased some, not much but some came out as I checked. Is there a connection between engine and gearbox? 

Valve cover breather and engine in overall is clean. 

Yes there is the rear crank seal behind the flywheel.

That could account for oil leak around gear box housing but not a raised level in the gear box

I think it's impossible to increase the gearbox oil from any leak... When the oil motor pass the crank seal, goes to the clutch that must be dry, then it slips and you can't use the car.

Sometimes the gearbox is filled over his level.

I think the oil goes from upper, maybe from the oil filter? and falls downward... outside the gearbox, not filling it.

Crank seal won't fill gearbox, only bellhousing.

I think people are reading too much into the first post.  When you remove the level plug on the gearbox, there is usually a little dribble that comes.  That doesn't mean the level has increased at all.