If you work with vegetable oils you can make your solution of DPPH in ethyl acetate, and also the different oils solutions. Ethyl acetate dissolve completely the hydrophobic compounds (nonsoluble in methanol) and results are also acceptables. It's important make your calibration curve of DPPH using the DPPH dissolved in ethyl acetate. The concentration of your samples depends of the ability to scavenge DPPH, you have to try different concentrations and find out the best range of concentrations that give you the points to fit your graphical DPPH, necessary to calculate de IC50 value. I work with a DPPH solution 6x10-5 molar concentration, by Brand method (Brand-Williams, 1995. Use of a free radical method to evaluate antioxidant activity), adding 3,9ml of DPPH solution and 0,1ml of your sample solution. But if your sample presents a very low kinetic sometimes the incubation time is longer than 3 hrs. So, It could be better work with a higher concentration of DPPH solution to reduce the time of reaction and above evaporation of solutions, that obviously affects the IC50.