Using oil...
Usually, the Northstar is just using oil, rather than leaking it out. The Northstar has a very aggressive cylinder hatch pattern, to promote sufficient oiling at high engine speeds (6000 rpm). Unlike many engines, this thing was designed for sustained high rpm use and will run at 6000 rpm all day. The aggressive hatch pattern tends to retain the oil longer than a slicker cylinder wall will.
Naturally, this oil retention keeps the oil up in the piston rings, which is good for lubrication, but it also means that it'll use more than "normal". GM has stated that 1 qt. for 1000 miles is satisfactory. We're used to a "good engine" not using any oil at all, but those engines also need rebuilds after 100,000 miles due to worn out cylinders (less lubrication means more wear). The Northstar is good for many many miles and the oil usage is a small price to pay for this reliability.
Having said that, there's also a tendency for the oil to find its way out of any small and minor imperfection in the block seal area. Not every engine does it. Mine never leaks onto the ground, but it is usually damp down there, meaning that a very minute amount of oil does seep out. It's no big deal and certainly nothing to be concerned about. Rarely does the case half leak drop more than a drop or two every night. If you have a oil leak that's producing a puddle on the ground, it's probably not the case half leak, despite what your Cadillac dealer will probably diagnose it as.
