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Discussion Starter #1 (Edited)
I finally got around to swapping the oil in my manual transaxle with Red Line MT-85 75W-85 synthetic. The main reason was to hopefully reduce the stiffness on extremely cold winter days, so obviously we'll have to wait to see how that works out. But here are some initial impressions other than temperature-related ones.

Smoothness of Shifting
The transmission was pretty smooth before, but shifting is a bit smoother now. The throw between the gears, however, feels ... not exactly stiffer, and not harder to shift, but smoother and "dampened," if that makes any sense. It's hard to describe. It feels "not klunky" might be a better way to put it.

I'm not sure that's what I want. But I'm not sure that it's not. Again, the winter will tell.

Noise
The improvement here was unbelievable. I never thought the trans was especially noisy: It sounded like pretty much any other compact-car manual transaxle. But now it's like silent. There's no whine at all. The change is also a bit amusing because I realized that I was shifting later than I usually do. I'd been using the whine as an aural tachometer and shifting cue.

I'm not especially sensitive to noise. Between having spend so much of life in airports and airplanes, my hearing isn't what it used to be; and having spent even more time in rooms full of computers with their random whirrs, beeps, and other noises, I've also become more accustomed to ambient machine noises. But if you're noise-sensitive, then this gear oil is for you. The transmission is so quiet that I take the car out on the road just to listen to the now non-existent transmission noise.

MPG
It's much too soon to say what, if any effect this gear oil will have on MPG. I will say that I happened to fill up today at a Sunoco station about 26 miles away. Because of an extended downhill portion toward the end of the trip back from that station, the car's MPG readout (which is actually very accurate in my Soul) usually reads between 39 - 40 MPG when I get home after just filling up at that station. Today it read 38.

However, I also made some stops in town while I was there (and after filling the tank), I had more cargo in the trunk than usual, I'm still shifting later that I did before, and I was stuck behind a mope (from Ontario, of all places) who insisted on driving a good 15 MPH below the posted speeds.

As for the late shifts, I find myself getting as high as 3,500 to 4,000 on the tach before shifting, which is extremely rare for me unless I'm climbing a bufu steep mountain or intentionally blowing out some carbon. The transmission really is that quiet.

I don't expect any major effect on MPG. It's possible it will increase slightly, but it's also possible that it will decrease slightly. The oil is friction-modified to GL-4 standards to avoid brass synchro damage, so the usual benefits of increased lubricity of synthetic oil are nulled out by the intentional lubricity modification. So if the oil is coating the gears better (which certainly seems the case given the noise reduction), there might actually be a small decrease in MPG -- but more than offset by increased gear protection.

But it's really impossible to say what, if any effect the gear oil will have on MPG without running half a dozen or so tanks of gas through her. My average calculated and indicated MPG (they're rarely more than 1 MPG off from each other) using 89-octane with ethanol has been running about 36 - 37, by the way.

Miscellaneous Oddity
I mentioned that I've been shifting later, but it also seems like the car's upshift advice is occurring later. That could be my imagination, though.

All in all, I'm glad I made the swap, but the winter will be the real test. In the meantime, I can't help but think that the absence of whining means the gears and synchros are getting better lubrication; so that in itself makes the change worth it, in my opinion.
 

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Discussion Starter #2
One other thing I noticed...

When you drive up and down the same mountains all the time, you get to know them pretty well. I noticed on two downhill stretches today that the deceleration due to engine braking is much, much less.

In the first case, the speed limit drops from 55 to 30 by the bottom of the hill. Previously I would shift into fifth at the top of the hill (from fourth on the uphill side -- it's a pretty steep incline), and I'd be at about 35 by the time I got to the speed zone ahead sign at the bottom. This time I was still moving at about 45 by the time I got to the sign.

The second hill is a much longer slope with a shallower incline and a 45 MPH limit because of the many curves and bends. Previously I could make the whole slope in fifth with no braking at all, and even would have to give a bit of gas here and there. Today I had to keep applying the brakes to keep the speed down.
 

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I'm a bit surprised it didn't come filled with synthetic to begin with. Back in '93 we bought a new Geo Metro (I was going for max mpg, drove 40-45k per year back then). After a year I figured I'd swap over to synthetic in the gear box. Draining it it came out lighter then motor oil and no gear oil smell, hmmm factory synthetic.

FWIW, that little green booger averaged 52-54mpg driving normally, and 47-48.5 driving like I was in a hurry. Took it real easy over two tank fulls and got up to 59mpg.
All my driving was 70-75% highway.
 

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Discussion Starter #5
I was a bit surprised, too. But I'm not sure that the oil was original. I bought the car "certified pre-owned" from one of the rare dealerships that I actually trust (because they've yet to give me reason not to), so maybe the dealership did a drain-and-fill as part of the prep.

I know for a fact that they changed the cabin air filter, which has NEVER been the case before when I bought a "pre-owned" car. It's one of the things I check for, and this was the first time I found a brand-new filter. They also did a complete brake job (pads and rotors) all around, replaced the tires all around, the coolant looks new, and the plugs look new. So I wouldn't be shocked if they also did a drain-and-fill on the transmission oil, as well. They seem unusually conscientious as dealerships go.
 

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Discussion Starter #6
An update: It was about 6 F (-14 C) outside this morning, and the car had been sitting since yesterday morning. The stick movement was nice and smooth -- barely different from what it would be on a warm summer day.
 

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Nice aint it! :)

Besides the Metro, I put synthetic in several other manual gear boxes as well as the transfercase in my old F-150.
Two biggest differences were in my old Trans Am. Had a Super T-10. Not sure if I used Red Line or Amsoil (back then I bought at a local speed shop, they carried both brands. I usually bought Amsoil but if they were out I'd grab Red Line). Either way, same weight, 75w-90(or 85). As you mentioned, very big difference in the winter, just about as smooth as summer. Other big one was my old F-150. It was a well worn work truck, hauling trailers and plowing. When it would get down to 0° I'd have to use my leg to push the stick into 4 low (this was a '79, no shift on the fly then) and I'm quite the opposite of a 98 lb weakling! After the synth oil, it was still stiff but easily a one handed operation.
I'm sure regular gear lube would have made some difference, when I drained the old stuff it came out almost like Lisa Douglas's* (Eva Gabor) coffee where you could cut it with a scissors.
It was a little quieter too, how much I'm not sure, it was an old truck with lotsa noises/rattles.

* For those of you old enough to remember Green Acres.
 

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Just a quick tid bit for people thinking of swapping out there MT fluid. Remove the intake before removing the outtake.

I got forked big time 10 years ago in my Miata. I drained the MT fluid, and then couldn't get the in screw out. It was seized like a mo-fo. I had to have it towed to the shop where the mechanic used a blow torch to get it off. I couldn't drive it cause there was no MT fluid in there.

$20 job turned into $200 tow and repair. :(
 

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Just to make sure that horse is dead 🤪, I put Redline in my car this morning. Bought it certified pre-owned, now has 132,000. Oil which came out looked like engine oil: color was weak coffee. After replacing with MT-85, the shifter has a very “slick” feel. And the car is noticeably quieter. Can’t wait to take it out on the road tomorrow!
 

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I used Redline MT 90 in the manual trans of my 1952 Plymouth and the change was dramatic. Smoother shits and no more grinding if you went to fast. Great product.
 
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