Airsoft Canada - maintanence on gun

I know your suppost to take apart your gun every 3000-5000 rounds shot and take apart the gear box to re-lubricate every thing. but is that all necessary? becuase on my AK-47 there is a little removable plastic peice under the trigger and that hole leads into the gear box. Same with my Aftermath knight, it has a plug that leads right to a hole in the side of the gear box. couldent i just put some oil in threw there or is it better to actually take it apart. get some dirt out and re- lubricate? another question i have which is really noobie but im not sure and that is, is it better to use liquid lubrication on the gears or more grease type stuff?