Since you bought it used I recommend you take off the recoil pad (it snaps off--twist it sideways). The take off the buttstock. You will need a 13mm socket with an extension. Pay attention to how everything under the bolt sits and be sure not to lose the lock washer under the bolt. When you take off the stock you will see the recoil tube. In older Benelli's this was made of Carbon Steel and had a tendency to get gunked up especially in hard use hunting guns and they would rust. In any case you need to clean the tube and the way the factory recommends is to spray in some gun solvent into the hole at the end of the tube. Swish it around and repeat a few times. Then LIGHTLY lubricate the interior with some oil. Benelli doesn't recommend you take off the end cap on the older tubes as its easy to bend them. The newer guns have aluminum recoil tubes and don't have the issues with corrosion. You shouldn't have to clean it very often--maybe once a year if you shoot a couple thousand rounds. Should be fine.
“Est Nulla Via Invia Virute”