It depends on the aggregation level. What is your nanoparticle and how do you disperse it? Simply, did you try extensive sonication for oil before measurements?

Aggregation level is 5 mg/l. 

Graphene

for graphene and CNTs, DLS is not very suitable. Because, these materials also exhibit rotational diffusion while nanoparticles show only translational diffusion. Consider, using AFM to see the level of aggregation. In any case, you may not get the accurate value, but essentially it will give you a sense of dispersion quality

Thanks Dr Harish, Do you have any idea  of using of Heptane solvent for dilution of this oil suspension? Viscosity of oil is 76 cst.