ntkb, the answer to your original question is that well made ammunition should last many decades before it incurs any noticeable deterioration. And oil should not be a factor, unless the gun is dripping with it. What causes the most problems with carry ammo is bullet setback from being chambered multiple times, which has already been mentioned. This is why I shoot my carry ammo about every six months. I don't want to have to worry about some kind of malfunction due to bullet setback. Anytime I clear my weapon I take the time at the end of the day to empty my mag and rotate the previously chambered round to the bottom of the mag. I know that's a bit anal, but it works for me.