The torque converter is fitted to automatic transmission equipped vehicles, and is a fluid damper fitted between the engine and transmission that allows the vehicle to come to a stop without stalling. It also provides torque multiplication, which effectively provides better low down response at slow speeds, and can also ‘lock up’ to increase fuel efficiency at highway speeds.
Any qualified mechanic or transmission specialist can replace the transmission fluid in your vehicle and they will ensure the correct fluid is used.
It is recommended to change transmission oil/automatic transmission fluid at the specific service interval as suggested by the vehicle manufacturer unless you have transmission issues or there are unusual circumstances that would require it to be changed sooner.
A manual requires human input to change gears, with the driver using their foot to depress the clutch pedal, and then manually shift the gear stick up into the desired gear. An automatic doesn’t require any of that, as it’s controlled completely by the computer in the car. All you need to do is tell the car what you want to do by putting the gear selector in Park, Reverse, Neutral or Drive.