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I have a PD4106 with an 8V71 and 4 speed manual trans.  I took the bus out for a short drive and when I returned home I noticed a stream of oil from where I pulled out of my driveway. I noticed the oil coming from an opening at the bottom of a black cylinder (approx 5 inch dia by 12 inches in length) located inside the engine cradle between the oil pan and the bumper. The cylinder has two hoses (one on each end) that connect to the side of the engine block.  I retraced my drive and noticed the oil stream when I made a turn.  It had stopped leaking oil by the time I got back home. What is this thing and why would oil be coming from it. I have the maint manual but it does not reference this cylinder.
Thanks

What you have is a collector for the air box drains, probably home built. It is intended to be emptied from time to time.  If it is coming out of the bottom, perhaps it is missing a plug.

If it is overfull then it hasn't been emptied in a very long time or you have excessive oil in the air boxes, perhaps from using the wrong oil or a lot of idling.

Empty and clean it all up then monitor for a while.

Removed

Examine the gunk when you empty it, it's a good diagnostic tool for possible failures.  should be black nasty oily stuff - no water or antifreeze!

Brian

If the tank is correctly built, it'll have a 'standpipe' sort of a drain bolt located in the bottom.  Any sort of standpipe is OK whether it's the drain bolt or otherwise.   Once enough oil collects, if the tank isn't emptied regularly, it'll drain over the standpipe.  That prevents blocking the airbox drains.   At highway speeds, airbox collector oil will cover your coach backside. 
A worn engine can fill the can quickly.  Regularly drain the tank and you'll get a feel for when it needs servicing.  Doesn't need cleaning, just emptying.
I need to check mine....sorta forgot about it.  ::)   
JR

Thanks for the help.  JR you are correct about the stand pipe.  I will empty it and check it regularly!
Thanks again!