Yes, it's possible that back-pressure through the oil drain line can cause oil to leak past the turbine compression ring back there...
...but we're talking about a stock turbo using the stock oil drain line, right?
Unless you messed up a gasket or went bonkers with the silicone RTV, that's almost impossible...
http://turbocharged.com/catalog/parts_list.html
Part #16 is the turbine compression piston ring that needs to be replaced.
Since these rebuild kits are relatively cheap versus the labor costs, you might as well get the entire turbo rebuilt.
-Ted