Oily Smell Inside Cabin

[mjt]

I'm getting an oily smell inside my 2010 Zetec 1.8HE. I usually notice it when I stop after a run although a few days ago I could smell it whilst running after slowing down for a built-up area. Popping the bonnet and taking off the engine cover I can't see anything obvious, nothing covered in oil, no split or disconnected pipework and the engine oil level is rock solid but the smell is definitely in the engine bay.

It seems to have become noticeable since its last service so I'm planning to take it back for the garage to have a look (I'm getting too creaky to drop the undertray and crawl underneath these days ) but I just wondered if anyone might have come across this before. I also wonder how the smell is getting into the cabin when the ventilation intake is at the base of the windscreen. The car's done just under 30K.

[IINexusII]

could be overspill stains on the exhaust manifiold causing the smell when the garage was filling in with new oil if you cant see any leaks

[mjt]

They did, in fact, overfill it slightly and I had to extract about 0.5 Litre to get it back to the top mark on the dipstick. It's covered about 2,500 miles since then and the level hasn't budged so I wouldn't have expected that to be a problem. I couldn't see any evidence of oil having run down and been wiped away from the filler. I did wonder if there might be a weep from the filter but I can't see down that far from the top so will have to leave it to them to check. It isn't a burning smell, more the kind you get when fresh oil is in contact with something hot.