Actually, it is quite typical. I've got dozens of examples to work off of. The exact flow pattern is based on one particular photo, but the all-encompassing black smear after that cowling line is quite normal on all but new airframes. As I'm skinning a particularly weary airframe I wish to include that kind of oil staining. The exact how is what I am trying to nail down. The exhaust stains are also noticable on a number of 410s. 410s had a lot of losses, and a lot of replacements. You find a new ride and it's clean. You see one that's survived a bit and they're dirty as heck around the engines.
The oil slick behind the cowling is even visible on some 110s in the exact same way, as a matter of fact.
The more I look at the exhaust stains, the more I hate them, however.
I think I got the exhaust stains on the nacelles okay (with some tweaking to help the points you mention), but the spread over the wing is making me sad right now. The texture doesn't look right. I'm rusty at this. There was a time I'd have nailed it in the first go.
EDIT: I am, in retrospect, considering reducing the oil slicks behind the cowlings simply because I can't get it to look the way I want. There would still be a long streak of oil if I do this but I'm thinking of making it narrower and not as flared at the trailing edge.