Buy a few O-rings when you get the chance, in time they will get nicked. Sportmans sell them by the hand full for cheap. As for the seal rings, I have over 45,000 on my set and it is still running strong.
Also, about the 30,000 mark, I got a new set of seal rings for my 1100. Remington sent me the new style rings (1187 chrome plated, the split ring snaps over the bottom ring and expands on the bottom ring to seal against the port collar) and tried the shotgun out. The shotgun kicked like hell, which I pulled the new seal set off the rifle and put the old style (bottom ring is solid and the top split top ring slides down the mag tube and mates to the top of the bottom ring) and have been happy ever since.
If you can get your hands on an Old style seal set, and you have the 1187 chrome set, get them. The recoil is a push, instead of slamming that the 1187 rings cause, due to too much gas opening the action.
Note: during cleaning, pull the carrier out of the receiver and clean the slide rods and receiver slots out, they can get packed with fouling (even worse if you use green dot reloading).
To remove, pull the barrel, then the charging handle (straight out), and then reach into the bottom of the receiver (with the trigger group out) and press in on the mag shell stop lever and pull the carrier straight out the receiver.
To reinstall, slide the carrier back in, then push in on the shell stop lever to allow the carrier to slip back into the receiver (on a 870, there is levers on both sides of the receiver retaining the carrier bolt into the receiver.