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I'm in the low 50k mile range now on my '15 6 MT and decided to change the gear oil -- why not, since it's just the oil and a couple of washers.

Observations: Almost exactly 2 qts drained out. My oil was not in bad shape at all; still had a decent oil-ish color and a bit of black material on the drain plug. I've heard some others say their oil looked awful in the 20-30k mile area; mine did not, it was fine. There was no reason in terms of oil quality to change it, in short -- at least on MY car I'd be comfortable going to 75k at least as an interval and perhaps to 100k.

2 qts, filling with a pump from the bottom, did not completely fill it. I filled on ramps, loosely put the fill plug in, backed off the ramps and removed the plug, expecting a bit to drip out (since capacity is listed as just under 2qts and the ramps are a bit of an incline.) It was not full, but was probably within the "safe" zone as a finger down the hole was able to feel oil. It took about another third of a quart to fill it all the way, and then when pulled back off the ramps and the plug was removed perhaps a half-ounce dripped out. The moral of this story: Buy three quarts of oil, not two, and if you fill on ramps to where it's full you won't have much come out when you pull the plug after backing off them. Also, the washers on the drain and fill plugs are aluminum and distort when the plugs are torqued; as such they are one-time use and if you try to re-use them they'll probably leak. Get some new ones from the dealer before you do this; they're cheap.

I am running MTL mostly because I've liked it before in manual gearboxes. I didn't notice anything materially different in shifting on a quick test drive after finishing up.

One final note (some of you may know this, but I bet many do not): Never run GL-5 gear oil in a MT that is specified for a GL-4 (as this one is.)

This includes oils that have both GL-4 and GL-5 specs on the label; if GL-5 is on the label DO NOT USE IT! And by the way if you look in the auto parts stores you'll find virtually all are either GL-5 or dual-spec.

If you ignore this warning you will destroy the synchronizers and wind up having to tear the gearbox apart to replace them; GL-5 oils have additives in them designed for very high-pressure environments that coat the internal parts, particularly hypoid gears. This is great except that the coating is of higher strength than the brass that the synchros are made of and as a result the synchro material will shear instead of the coating under load and soon you will have no synchros! The moral of the story is that a higher number is not always better and if the GL-5 designation is on the label do not use it in a synchronized gearbox unless the manufacturer specifically says it's ok, even if the other (e.g. GL-4) spec is present on the label as well.
 

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Lots of great info here ticker - added to my 3rd gen 1 stop shop.

Side note: for the AT, Mazda doesn't recommend a change until 80 - 100k miles; though doing it this early won't hurt a thing.
 

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I just swapped my gear oil for the MX5 (recommended at 30k, did it at 35k) and did so with Motorcraft XT-M5-QS. It's great stuff, recommended by lots of Miata folks, as well as Speed owners like SonicNutrition. Idk if its good for 3rd gens but its worth a look at it.

Clicky

It's great stuff, really smoothing out my shifts, and in addition to my new Sure stainless knob, it feels like a whole new car.
 

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I'll add that you need a 23MM socket to remove the two bolts.

Also you can gravity fill from the top with the hood open if you remove the factory intake box. Just get clear plastic tubing from home depot and tape it to an oil funnel. This will allow you to fill the oil with the car off the jack stands.
 
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