Thanks chaps!
The oiliness I created by mixing Humbrol steel MetalCote with matt black, then buffing it up a bit. I tried to apply it where boots would traipse it while working around the pump equipment. The shack has only had grey primer applied, then weathered down with dry-brushing mucky colours, and also a bit of grungy washes.
I sent some snaps to the client and got an email back with some positive words in ALL CAPS and several exclamation marks. I guess he's happy!