Concentration of sheens
One question was on concentration. If you see a sheen, how do you know how much oil there is?
@eustatic mentioned the Bonn Agreement on Oil Appearance Code: mentioned in this oil sheen thickness guide by NOAA: http://response.restoration.noaa.gov/sites/default/files/OWJA_2012.pdf (see lead image as well)
Concentration's affect on spectra
@ethanbass mentions tests he's done (and research he's found) that shows spectral change from different dilutions of diesel. But mentions the idea of looking at the darkness/opaqueness (using absorbence spectroscopy) of the sample as a measure of its concentration, which we could use to adjust for concentration in our tests.
So, if you have an unknown, at a certain opacity, you need to get your reference (crude oil, say) to the same opacity to make a comparison. You could use absorbence spectroscopy to achieve this, perhaps. (@ethanbass)
Oil absorbent cloths
Oil-absorbent plastic cloth ($79 per hundred sheets): http://www.newpig.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10651&catalogId=17208&langId=-1&partNumber=p_MAT403&cm_cat=PPZ1
This page says that the cloths "Can be wrung out and incinerated after use to reduce waste or for fuels blending", and are made of polypropelene.
More in the comments, coming!