I actually think it was the heat gun I used that finally let it loose. Have you tried one of those?
I’ve bought this from Bofi to replace the square plug.
Jass Performance Hex Gearbox Filler Plug for Mazda MX-5 NA NB (bofiracing.co.uk)
I did the gearbox and diff oil change today. The diff drain plug needed the breaker bar and jack treatment to get it off. The diff only took about 0.5 litres instead of 0.65. I had the car level.
Now that I know how to overcome these problems I might be able to use up the 2 quarts of 2010 Redline MT90 which I’ve had sitting in my garage since my Eunos days.
Assuming it’s compatible with a 12-plate mark 3 (?) and is still within it’s sell by date.
I have mt90 in my nc 6 speed. Lovely when warmed up, still a bit notchy when cold 1st to 2nd
Worth a go…
Your fill plug if 6 speed will be an hex type plug head, the 5 speeds had the square head fill plugs as shown above.
Sorry I meant try an hex type socket.
I remember having bother with this type of plug before, think on a Mk1 gearbox. I managed with a pair of stilsons to move it, that’s the ex plumber in me, still have some of the tools.
Going slightly off topic but I’ve stumbled across this old thread discussing different oil types. Some useful stuff (and pics) in there.
I managed to get out this evening and succeeded in removing the fill plug and draining the oil (but I’ll have to re-fill it another time).
I just couldn’t get the 14mm spanner to work, but I discovered (after I got the plug off) that what it had been cleaning the plug of some of the gunk that was stuck on it.
After failing continually with the 14mm I tried a 16mm socket again on the off chance and this time it went on (because of the gunk removal being done by the 14mm spanner).
I still couldn’t get any form of leverage on the ratchet handle, and then I had a brain wave - could I put my leg under the car and kick the handle (bearing in mind the car is still on the ground on all four wheels).
Turns out, Yes! and three kicks later (with the handbrake on) and the fill plug came undone.
Surprised you were able to get under the car without jacking it up.
I’d already done the engine oil and diff oil with the car on the ground.
Engine drain plug was the worst of all of them, it required a full 24" breaker bar to release.
Not managed to replace the oil filter yet, got one of those chain wrenches but it just slips on the old filter (but try it on the new filter and it grips brilliantly 
).
Are you still having an issue with the fill plug? i have a solution, but never use a open spanner on it, it will slip and round it off causing bigger problems.
M-m
The original poster sorted his with the use of a heat gun.
As I was having the same issue at the same time as they were, I’d been sharing the thread and yesterday posted the method I found worked for me.
All the forum guides say either a 14mm open spanner, or a 16mm socket - are you recommending against the 14mm spanner then?
I believe it was your site that had an open ended spanner with a welded top bar, was this the solution you were referring to?
100 % do not recommended an open spanner, we have dealt with many many aftermaths of them being rounded off to be removed by those that know best than to listen.
The one on menders is an open spanner that’s had a very thick run of weld so it becomes a lock on square key.
Heat , you need to be very ,very careful, never had the need to and never will.
When they are rounded after jelly hands as tried their best to re form it int anything but, we use a universal socket from wurth, but there are other makes like US PRO… US PRO 19PC 1/2" DR GEAR LOCK SOCKETS 8 - 32MM MULTI FIT SOCKET 3221 5060478385394 | eBay
and so on.
M-m
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