does changing gear oil help improve condition of gear

my gearbox makes noise when i go into 3rd gear and continues when i am in 3rd gear and also when i go into fourth.if i change the gear oil do you think there will be any improvement to the gearbox.

also my timing chain makes noise when i have driven for for a sustained length of time,or say after an hour or two of intermittent driving it starts and continues but i hear no noise when i go on short journeys at the beginning of the day.does it need to be changed immediately or can i wait till the time when i hear noise continously???