You can use Flame Ionization detectors to find out the HC content is waste oils in which a sample of oil is burnt in an electric field, +ve ions produced are proportional to the carbon atoms.

These are popularly used for UBHC (Unburnt hydrocarbons) characterization for automobiles.

If you need to discriminate between new and aged lubricant oils, I may suggest to use IR spectroscopy and elemental analyses (based on ICP-OES or ICP-MS and CHN analyzer), as your lubes contain also a significant part of additives (meaning that you will have a significant content of heteroatoms). You will obtain a first useful fingerprint by this way. For a second step, you can also consider GC-FID to compare the evolution of the hydrocarbon chain length of the base oils (not applicable for additives) in addition to the fuel dilution. 

Please clarify your question.  If your goal is to first determine whether crankcase oil is "fresh" or "used" or even "overused", you need to take into account the impact of the additive package.  Oil drain intervals are often selected based upon a comparison between total base number (TBN via ASTM D-4739 or D-2896) vs. total acid number (TAN via ASTM D6649).  These are wet-chemistry, titration-based analyses and the results are in units of mg-KOH/g-lubricant. TBN depletes and TAN increases with engine operation and oil change interval is sometimes specified to occur just prior to the point where TBN=TAN.  You should be able to determine an index of how "used" an oil is based upon a comparison of the TBN:TAN titration results and then conduct a more detailed analysis of the hydrocarbons.  Keep in mind that HC compounds in oil can change via oxidation, polymerization, and thermal cracking depending upon the application and the operational conditions that the oil is subjected too.  HC compounds in crankcase oil can also change due to fuel dilution of the lubricant.

Dear Joseph,

Thank you for your response.

The goal of the characterisation is not to determine the status of the oil-in-use but, to determine its content prior to being used for studies on car park pavement structures. We intend to do material balance at the end of the study and so, need to know some of the parameters such as heavy metal content, HC content etc. prior to the commencement of the study. Further information will therefore be appreciated.

Thanks.