Do the spectra of compound materials combine linearly with respect to the ratios of the constituent compound spectra? My question stems from reading this article on the FTIR spectrum of oil that has been mixed with various contaminants.

If one has the spectrum for the original uncontaminated oil and the spectrum for water, will the spectrum of a sample that has been contaminated with 10% water (by weight) be equivalent to the addition of original oil sample spectrum and 0.1 of the water spectrum intensities?

(Also, should the unit of contamination be the weight or the volume?)
Yes, you are right. If you have $n$ components in a mixture each with a spectrum $S_1$, $S_2$,...,$S_n$, the observed spectrum $S_{obsvd}$ would be the sum of $S_1$+ $S_2$+...+$S_n$. 

In FTIR you either measure the transmittance or the absorbance. The term intensity is more or less used when you deal with molecular or atomic emission of light (fluorescence etc.) As long as there is no chemical reaction among the constituents, the absorbance should be linear with the concentration.