hey good job learning new things for your car. the process is simple for a 307 .. i have a 307 too and i dont know if other peugeots have dipsticks or not but this is how i check the oil in the gearbox ... or have it checked basically.
- Park the car on a flat surface.
- Raise the car with a jack or something. you just need to access the gearbox level plug.
- Place a pan under the gearbox level plug so that you can catch the oil.
- Start the car and wait for it to go idle
- Hold the drain plug in place with a combo-wrench and remove the level plug with another combo-wrench
- The flow of the oil till tell you how much oil is present in the gearbox... if it flows and then drip towards the end then the gearbox has enough oil. If there is little flow and starts to drip quickly or no oil comes out then you need to fill it up.
- Reconnect the level plug and turn the engine off .. open the bonnet, remove the air filter, DC the battery and remove the battery tray, locate the filler plug on the top of the gearbox, remove it after loosening it, insert a nozzle and add 0.5 lt of the fluid. Reattach the plug and repeat everything to insert things in again.
- Remember to connect the battery again.