Hmm, I'm not aware of the oil based paints damaging silicone, although that doesn't meant to say its not true. If the oil based paints contained thinners or another solvent, then possibly. However, if this is a concern, simply use electrical or masking tape over the silicone to protect it. Use newspaper over the sides of the tank (to avoid spray deposits).
It would be a good idea to turn the tank upside down, but again, cover the top with something first, just to avoid any mist landing on the front/inside of the tank. Perhaps rest the tank on an old towel or something. This will stop the spray from hitting the ground and blasting forwards, underneath the rim and into the tank. The last thing you want to be doing is rubbing paint flecks off from the inside of the tank!
Make sure the glass is completely free from any dirt or oil. Use a window cleaner on it first, or hot water with white vinegar in it. Use a clean, lint free cloth to polish it. Then commence with the painting. Hold the spray about 18" away from the surface of the glass and do short bursts, left to right in a sweeping motion. With spraying I've found that its best not to continuously hold the spray down but rather, press whilst spraying left to right. Wait 1-2 seconds, and spray again right to left. This stops the nozzle from clogging and the paint from "spitting" out. Let the paint build up gradually.
Make sure you get enough cans to do the job, and it might also be worthwhile investing in a matt lacquer. This will safeguard the paint against scratches once its dried. Believe me, there is nothing more annoying, than getting a perfect paint finish on something, only to have to scratch later on by accident.
Wear a mask and goggles. Stand upwind of the object being sprayed and wait for a day when the wind is very calm.
HTH and good luck