This is an updated set of simulations showing the voltage stress on the dielectric in a capacitor section. The geometry is a 20 mil gap on either side of a 60 mil piece of poly with a dielectric constant of 2.2. Three simulations are, one with air in the space, two with oil having a dielectric constant of 2.0 and 4.0. The electrodes have 15000 Volts difference between them (+7500V to -7500V). The poly is stressed the least (101V/mil) when air is in the gap. This will, however, cause corona and be destructive to the dielectric. The next case is with an oil having a similar dielectric constant to the poly. The stress is 142V/mil. Finally a higher dielectric oil is used and the stress is 182V/mil. If there is enough poly the handle the voltage then the higher dielectric oil will get the most capacitance from your poly. If there is any weak areas in the poly this could however cause a voltage breakdown. If the gap is small compared to the poly, then any oil is most likely going to work and these effects will also be small. For this example the capacitance (related to charge/m^2) and energy stored in the poly in each case will be as follows.
Air (1.0) 0.07*10^-3 C/m^2 155J/m^3
Oil (2.0) 0.11*10^-3 C/m^2 314J/m^3
Oil (4.0) 0.14*10^-3 C/m^2 505J/m^3
The higher the dielectric material that is placed in the gap the more it is like placing the electrode directly on the surface of the poly, as if the gap is not there. Any imperfections under this condition are more likely to fail. Using an oil with the same dielectric is like smoothing out the surface of the poly and any imperfections will be more tolerant.