Because oil is ususally so volatile there is at a risk of it not burning quickly enough and then you get methane and other incomplete combustion products.
You have to slow the burn down by absorbing the sample into an inert material - inside the cup.  A lot of people use Chromosorb.  We have had good success with Whatman GF filter paper but it depends on the viscosity of the oil - whether it will absorb it.  Heating it may help.
Steve Price
Pelican Scientific
Chester Road
Chester CH3 6HJ
Tel +44 (0) 124 467 0177
Fax +44 (0) 124 467 0171
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