The optimal dilution can vary depending on the concentration of IL-6 in the serum samples. You should ensure that the diluted sample fall within the linear range of the standard curve for accurate measurement.
You may try administering high fat diet for 6-8 weeks rather than 2 weeks.
1:2 dilution may do. However, this is dependent on the total volume of sample required for the ELISA. You may find yourself using either 50 or 100ul of serum or plasma.