I would assume you need only apply it to one face (in this case, the part of the oil can resting on the plate.) But I don't know for sure, maybe one of the VAR guys can chime in on this.
Why not run a simple experiment, simulate heat conduction between two cubic prisms and apply your contact resistance to one face only. You can sketch a line through the centers and use the XY plot to see what your temperature looks like.
OK so I got curious and ran this simulation experiment myself. Two cubic prisms, 100MM on a side each. 1 W of heat transfering through, with a 1 K*m^2/W resistance in the middle. The heat flux through the resistance is 100 W/m^2, so the temperature difference should be 100K, and indeed it is:
So in short: use just 1 face for your contact resistance.