That method works for twenty round mags.
Thirty round mags have a different style floorplate. You have to take a small drill bit or similar shaped rigid tool, insert it in the hole on the floorplate, and gently pry to lift the end of the plate just a bit. While your are prizing the end of the plate up slightly, push it toward the edge of the mag. Once the little intents clear the edge of the mag, the plate will slide off.
Getting the follower in and out is a bit of a trick, too. You have to tip it sideways a bit to clear the tabs that hold the floorplate on. Do not bend the tabs, they will break off if you try to straighten them to "get them out of the way".
You shouldn't oil the inside of the magazine. Just wipe it clean and lightly oil the spring. Excess oil in the mag will collect dust and dirt.