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Two weekends ago I changed the MT gear oil in the 2017 Mazda 6 Touring MT. I had about 9k miles on the car at the time.

The manual in the car was fairly notchy, especially into 2nd, so I thought I would try to smooth it out. I filled it with 6 oz of LubeGard Gear Fluid Supplement and 1.6 quarts of RedLine 75w80 synthetic. RedLine is one of the few premium brands to make that weight of gear oil, which is why I chose it so I could stay within warranty spec.

I'm glad I did it, because when I drained the old oil it was still translucent, but it had a bunch of metal flake in it from breaking in. The result was that the car does shift much smoother. I have driven almost 1k miles since and the shifting is getting progressively smoother. Even 2nd gear is sorted out, I can shift as fast as I want and not worry about grinding into gear.

Once the car is out of warranty at 120k, I will probably switch to the less economy-minded 75w90

Props to RedLine, they make some killer gear oil.
 

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SF0059, I want to thank you (alot) for posting this very useful info. I too, have a '17 Mazda6 6MT, albeit a Sport (not a Touring). I also want to thank others in the 6MT forum division, for posting info on early model year Series 3 Mazda6 manual transmission ills, in like manner.

My sense on my 2017 6MT, as expressed here (http://forum.mazda6club.com/newbie-section/415377-general-impressions-2017-sport-6-mt-300mi-odo.html) is that all of the 6MT's (or at least many of them) exhibit some 2nd gear synchro roughness... and so your change to Redline with your addition of the additive, is very instructive to me. I think this transaxle, simply, is this way - by way of design.

Obviously you have had experience with the additive, too. I wonder if straight-gut Redline 75W80 GL4 synthetic, alone, would have quite the same curative effect.

On a related note, the Italians call the wear-in material "rodaggio" (not sure re the spelling) - and it is really typical of a geartrain that has to wear in to fully be happy to mesh... lol. I was thinking of getting a rare earth magnet type fill plug and drainplug, to substitute for the OEM plugs. Reason for the fill plug, too, is that the "rodaggio" can also float on the surface... and so you can remove more of this with both fill and drain plug having these powerful magnets.

The downside (many mods have downsides :frown2:) is that if the adhesively fixed magnet (which maybe doesn't only rely on the adhesive to stay in place)... COULD POSSIBLY become detatched, that would be bad. Also, you would have to be sure that for each gearshift position (including reverse) no possibility of an internal component "fowling" said magnet which reasonably has to protrude/penetrate to the inside of the gearcase in order to work reasonably well... would ever occur.

Here is one source of these magnets: MAGNETIC DRAIN PLUGS

Cheers, and again many thanks to you for posting your info, your experience with your (relatively rare) 6MT 2017 car...

Cdn17Sport6MT - Vancouver, BC
 

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I put Redline MTL in my 6 Sport around 50k ago and it did help materially with the notchiness, especially going into 1st at a light. I've been happy with it.
 

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I never tried their 75W90NS in my Mazda manual tranny, nor any other of my manuals that called for an 80. Is there a specific problem or irritant you're looking to fix or change?
 

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I really would suggest no higher than 75W-80. The C66M-R has some hollow shafting through which the lubricant is "pumped" to supply the speed-gear bushing-bores. Higher viscosity would reduce lubricant flowrate; not great when you're lubricating moving parts. Also, I've read that NS is h*ll for synchro's.
 

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That 75w90NS is a GL-5. DO NOT RUN IT IN A SYNCHRONIZED GEARBOX THAT SPECIFIES A GL-4 LUBRICANT, it will destroy the synchros.

NO GL-5, no matter who makes it, should EVER be used in a synchronized gearbox unless the manufacturer has specifically listed GL-5 lubricants as acceptable. There are a handful of specific manufacturers and gearboxes where it is acceptable and even recommended (e.g. certain Porsche gearboxes) but for nearly all manufacturers, including Mazda, it is not.

It does not matter if any other spec is on the bottle; if GL-5 is on the label and the spec for your gear oil is GL-4 do not use it. Period.
 

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I really would suggest no higher than 75W-80. The C66M-R has some hollow shafting through which the lubricant is "pumped" to supply the speed-gear bushing-bores. Higher viscosity would reduce lubricant flowrate; not great when you're lubricating moving parts. Also, I've read that NS is h*ll for synchro's.
I think that’s a good thing to keep in mind for viscosity choice.

The NS product’s frictional characteristics are correct for a synchronized manual transmission. Note that Mazda indirectly calls for a lower EP treat rate than what the NS product uses. I say indirectly because they simply call for a GL-4 gear lube, but the way a GL-4 gear lube is made is to use a lower treat rate of the exact same additive system used in a GL-5 lube. It is quite possible that what they really want is a completely different product; one with the proper dynamic friction characteristics for synchronized manual transmission use (like various Red Line products). That is what I would use. They don’t specify that, though.

So, Jake Wilcox, if you want to play it safe, stick to the 80-grade Mazda specifies. They also offer an 85-grade, but again the pertinent question is: Is there a specific problem or irritant you’re attempting to eliminate with a higher viscosity fluid?
 

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OBTW, if you don't believe the formulating chemists concerning whether a specific fluid is suitable for all synchronizer materials, you could always try MT-90. Still curious what you're trying to address or accomplish.
 

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If you have a case where the alternative lubricants you are considering are all very similar by way of being tailored to the correct frictional characteristics for synchromesh operation: then i) lower viscosity will generally give you smoother synchromesh feel; and ii) higher viscosity will give you more min. oil fim thickness (thereby giving less gear tooth flank wear). Anti-wear additives also figure-in too, though. These are always competing priorities. Within the mfr-stated 75W-80 GL-4 requirement (I think SAE 306 spec?... the version of the spec at the time that the design of the gearbox was done) - there is a range of Kinematic viscosities (cS) at +40°C and +100°C values and a range of Dynamic viscosities at -40°C (cP) that are permissible within the ranges of the spec. I THINK I've got that right... Unless you are really running the car hard, in high temp conditions, generally you would want to (I think) go towards the lower viscosity lubricants. No longer mfr'd... but still available if you look hard... and expensive - would be Pentosin MTF-2... (I THINK that is the correct product designation) which for me would be an optimal 75W-80 GL-4...
 

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BTW, if your synchro action isn't really smooth - and you are attempting to cure it with a fluid change - please note that (for me) there are at least four other things you should check before you condemn either the fluid or the design of the transmission (or figure that there's something internally wrong with it).
 
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