Re: Auto or manual
When driving an automatic you should always leave it in "Drive", or whatever the forward gear is called on your car.
When you shift from neutral to drive, brake bands are used to lock elements of the gearbox. These are like any other brakes and the more they are used, they more they will wear. Usually this is not a problem but if someone is shifting from drive to neutral to drive at every traffic light or stop this will increase the chances of wear.
When in "drive" the torque convertor (fluid clutch) is transmitting a small amount of power. I just keep my foot on the brake. On modern automatic cars when a gear is selected the idle speed will drop slightly. This is a ploy to save fuel and is not because the engine has come under a lot of strain. Hence the fuel savings shifting to neutral will not be huge (despite what your friendly taxi driver might tell you).
By the way, I only ever use my right foot for braking. The first time you try using your left foot, you'll realise how much work your left leg has to do to press a clutch.