Don't forget to lube the barrel lug as well.
Functionally, you probably can't over lube a gun, but it can produce some nasty stains on your clothing, face (when firing sometimes) and such.
As for how much, really you don't need much at all, usually no more than a drop at each location and a bead down each rail should be sufficient. When you re-assemble the gun, work the slide several times, wipe any excess, and you should be good to go.
Keep in mind that for summer carry, you probably need to relube your gun once a week if you are outside very much. Many of the lighter oils dissapate with the heat. In winter time, you may want to avoid grease as grease can become thicker with colder temps and adversely affect the functioning of your gun.
One advantage of grease for summer carry is that you won't see any lube draining out the end of your gun and staining your holster like you will with oil if you get too much on it.
I have become a fan of lithium-based grease because it lubes well and I have not had problems with the gun running dry even after carrying the gun for an entire week and shooting 500 rounds. I have not been that lucky with oils.