Leave the oil in it and you'll be just fine. I wouldn't use mouse/rat killer inside the car as they will eat it and crawl off to die inside your car somewhere = NASTY! I would use traps outside of the car at least that's what we did and found mouse/rats in them occasionally. FWIW we stored our Firebird for decades and have never had problems. Left the battery in it unplugged and never poured oil down the cylinders. If your were going over a year I would but you said only four months which isn't that long. My Firebird has its original engine that has never been apart and it does not burn oil.