Pressure on a seal prevents the "dry" air from robbing the seals of moisture, so an empty mag will crack and ruin if left without the gas in mag. The oily gas is just a preventative measure by the paranoid, but it works the same as shooting oil'd gas (the same amount you put in to shoot regularly).
Personally: I dump the rounds from the mag a day or two after the game (when I remember, otherwise I do it on the way home in the car or when I unpack at home). But I make sure my mags ALWAYS have gas in them, this is something to be paranoid about, and takes two seconds to do before you PACK your gear when you leave the field, or after you shoot at home. Never leave mags empty on gas.
Also something I never thought about, because I don't do it: DO NOT store mags in a hot environment (car in direct sun) or in direct sunlight. This has caused mags to over pressurize and release the gas (not explode... this is why we have seals etc... they act as a safety on the mag from explosion). After the mag is finished releasing gas... depending on what did what... You may have a mag, empty, sitting in the sun, with destroyed seals. :\
Edit: Leaving silicone oil on the seals WITHOUT pressurizing the mag could also ruin them. The pressure from the mag prevents the seals from over expanding from the excess oil... An over expanded seal may be compromised and not seal properly when the mag is filled with gas next.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.