Yep, that oil stain. Why don't you create an oil stain with metaballs? It will take time, it will probably increase render time, but it will certainly look better. Just flatten them to ... 0.1 BU or so, and then put them so the upper half is above the pavement surface. Copy and change X-Z dimensions to taste. Even the part that flows down into drain can be easily done with metaballs. And possibly you might get that oil/water effect in the drain. Now I'm already hairsplitting, the grille doesn't have a noticeable shadow. Which would be a pity to remain. It's a great model and especially the textures.

-- erlik