Originally Posted by wpoll

Hey Sean,

The metal box thing is an oil cooler - oil is diverted through the core of this unit (correctly called a heat exchanger) and coolant is run through another set of galleries in the core, cooling the oil on the way. There are four ports on this puppy; two oil (in and out) and two coolant (also in and out).

The oil being cooled is sometimes engine oil, sometimes transmission fluid - depends on the exact model you have.

Have a look at this: -

RealOEM.com - Online BMW Parts Catalog

Your leaky seal might be part #12 - the x-ring (like an o-ring but it has an "X" profile).

HTH.. :-)