Residue under fuselage from breather?

I own a 2004 DA40 (my first plane) and on the left side under the
engine is a breather tube and some sort of overflow line for oil. I
noticed pushing the plane in yesterday that there is a sooty residue
under the airplane, only on the left side. I went an looked at another
DA40 of the same year and maybe a little less flown and didn't see
nearly as much of the blackish soot underneath.

I've got two questions:

1) Is this normal?
2) What may have caused it?

I've been flying in the last six weeks cold weather, and Diamond
suggests runing for five minutes after start at 1500 RMP, however I
usually lean it back about an inch (of travel on the lean lever) after
it starts, and during taxi to the run-up area. I had the oil changed
about 40 horus ago and am runnnig winter rated AeroShell.

Anyone have any thougths?