Transmission oil change procedure for a Caterham with Mazda gearbox

The following article is courtesy of @CBuff and was repurposed from his post below.  If you have questions or comments about the procedure, please comment in that thread. 

Caterham 420 5 speed transmission fluid change

Figured I would share. I’m a total mechanical novice so doing a transmission fluid change is a bigger undertaking than it should be. But I think it was successful. 
 

new 2023 caterham 420r delivered last June. Put 5k miles on it since last year and 7 track days. 
 

other forums and service suggest checking your transmission fluid level at the end of the season.  The only way I could figure doing this was to remove the filler plug and stick my finger in there. Since that’s the hardest part of the job I figured I would just drain it and put in new fluid. 
 

checked the square fill plug (9/16 crescent) to make sure it comes off.  

 

then removed the 24mm drain

I pulled back the carpet and cut the heat shielding that covered the hole in the footwell. 
 

nothing spilled out when I opened the filler hole.  About 1.5 liters came out. Black and nasty for less than a year.  Mag plug had decent amount of crud on it 

put about 1.8 liters back in via a funnel and tube (cheap amazon). When I went to check and pulled the funnel it was dripping out of the fill so guess that’s full. 

decided I should put a grommet( 60cm with a piece cut since the diagonal cross brace cuts thru the hole) back in the foot well and the heat shield plug (taped from the trans side).  

heat shield tape to cover plug

stick carpet back down

I started to wonder about the turret oil. Some Miata forums say to service this by pulling the shifter out. Decided I would see if there was any oil in there.  (Sorry no photos of this) 

pulled the tunnel cover. Removed three 8mm bolts on shifter. Removed shifter (careful it drips trans fluid). Seems like there was a little fluid way down at the bottom. Thought I would top off.  It took about 200ml to fill it just over the bushing receiver ). I read it should take 250ml from empty so it was either very low or I overfilled it.  
 

here is a video of some doing this in their miata

took it for a test drive and nothing was leaking etc. so success for now. Sometime down the road I will extract the fluid out of the turret but I was pushing my comfort zone and thought just topping off was enough.
 

so I can add to my first list. Transmission fluid. (Already did the oil twice, and diff fluid ).  I enjoyed the experience and learned a bunch.  Starting to get just enough knowledge to be real dangerous.