Change Gearbox Oil

[shnazzle]

Changing your gearbox oil is a highly recommended action for any car over 60k miles and is very easy to do!

Materials
- 75w90 GL-4/GL-5 gearbox oil. Redline MT90 recommended but pricey. You'll need 3 bottles for a 6 speed and 2 for a 5-speed
- 24mm socket
- 10mm Allen key (6-speed only)
- 17mm socket (5-speed only)
- Funnel
- 2ft hose (garden hose will do, just)
- oil pan

Info
A) the "plug" that is visible from the engine bay is best ignored. It can be filled using that hole, but it's not what it's meant for and it makes for an imprecise filling. If you don't know what I'm talking about... I never said anything.
B) Drain plug in the massive flat 24mm nut on the lowest point. There is no alternative.
C) There are two holes you use to fill the gearbox, depending on your gearbox. And the relevant hole for your gearbox is also how you know when it's full.

Left hex key nut (Allen key) : 6-speed
Right nut at the far right of the case (spanner or socket) : 5-speed

Steps
Prep: have the engine be warm (not hot)  so the oil drains out better.
1. Remove plastic undertrays
2. ENSURE CAR IS LEVEL (or at least when you're filling the gearbox up)
3. Check that the appropriate filler hole for your gearbox can be undone
4. Place the oil pan under the large 24mm nut on the very bottom middle of the gearbox, facing to the side of the car.
5. Undo the drain plug and remove. Let oil drain
6. Put drain plug back in. Does not need to be tightened to death! You don't want to crossthread this.
7. Connect hose to funnel
8. From engine bay, feed the hose down past the right of the battery down to the gearbox
9. Undo the relevant filler hole and put the hose end in it.
10. Pour new oil down funnel. When approximately the right amount is in for your gearbox, remove the hose from the hole and check if the new oil comes out of the filler hole. If not, hose back in and keep filling until it does.

 The filler holes will only let you fill up to the factory specified amounts. So, depending on how much oil is stuck onto the gears, that can be anywhere from say 1.7 to 2.1L depending on the gearbox.
What matters is that you fill the gearbox using the correct hole (as above) until it starts to drip/slow stream out of the same hole.
What does that mean? It means it's filled to spec. Could be 1.8L, 1.87, 1.94, 2.0...

If you want to be 100% precise. Make sure the car is perfectly level, as in, totally. Fill it until it starts to come out. Leave the filler plug open for 20 mins or so for all the oil to make its way off the gears and into the case and any excess out the filler/level hole.
Then you're 100% sure your car has exactly the amount it needs. Unnecessary to be this precise.
If the car isn't level, this can affect the fill amount! (if the car is tilted a few degrees to the left, the 5-speed filler hole will take more fluid than it's supposed to.