Grime in the heater parts, design flaw? Options

[rjames]

Edit to make sense :-)

So as I am in the middle of backdating my exhaust system, it was hard not to notice how much oil/grime was in the heating tubes (everything that connects to the exchangers and heater hose under the car).

On each side there is a connection to the fan houseing and I am assuming that that's where the grime comes from. Seems like there should be some kind of filter or something to keep the tubes from getting filled with crap.
At least that's how I felt after spending a day trying to clean 'em out.

[rjames]

Actually the reason I posted this question was due to how messy the heating parts I obtained from someone else were. The heating bits I took off of my car weren't exactly spotless though either. I know I have an oil leak (or two). I'll probably take some photos at some point and see if someone can help me diagnose where it's coming from. Unfortunately I don't have the space at the moment to drop the engine to really clean things up.