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Old 11-04-2020, 10:20 AM   #1
SightsSetStudios
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Default Heavier gear oil to stop synchro grinding?

My 5mt has recently started to grind a bit in 2nd and 3rd. I've noticed it only does this one the car is warmed up though, like after 20 minutes of highway driving. I have 75w-90 in it right now. I'm wondering if I can mitigate the slippage by moving up to something with a higher viscosity at temp, like maybe 75w-100? Or even higher? I live in California so it never gets very cold and keeping the winter viscosity the same everything should be the same in cold weather til the car warms up in theory, correct?
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Old 11-04-2020, 11:01 AM   #2
Samurai Jack
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What brand 75W-90 are you using? They are not all the same.

What I used in my 5MT was a 50 / 50 mix of Redline 75W-90NS and Redline Lightweight Shockproof.

5MT ran much smoother. Ran that for years with zero issues.
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Old 11-04-2020, 11:30 AM   #3
SightsSetStudios
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Penzoil synthetic
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Old 11-04-2020, 07:24 PM   #4
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Do you think switching to Redline would stop the grinding, or would a heavier oil be the best bet?
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Old 11-04-2020, 08:03 PM   #5
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Well, it all depends on what you mean by " grinding ".
Everyone's idea of what it means differs.

If this is a little grinding when shifting, what I used will make it go away, unless you just aren't shifting properly which is causing all the grinding.

If you are heaving massive grinding, then probably nothing will help, except a new / rebuilt transmission.
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Old 11-04-2020, 09:26 PM   #6
SightsSetStudios
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It's not my shifting. It's just some trouble getting into 2nd and 3rd when it's warmed up. Like it's bouncing off the synchros till it finds the right spot. Only lasts about a half a second.
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Old 11-05-2020, 11:06 AM   #7
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Just not trying to spend $100 on gear oil if it won't fix the issue. I just replaced the oil about 6 months ago.
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Old 11-05-2020, 11:37 AM   #8
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I have tried MANY different gear oils in Subaru transmissions over the years. I have been most happy with Motul Gear300.

Now, with that being said. If you have a synchro that is just a little catchy and not engaging smoothly a good gear oil will help, BUT if the synchro is actually physically damaged or worn out then no gear oil in the world is going to help that.

Sadly, the only way to know for sure is by spending money. The expensive way is pull it, tear it down and inspect, and since it's apart you may as well just replace the wear items anyway. The less expensive way is spend the $100 on good gear oil, BUT it's a crapshoot. If it's better then great, but if it's not then you're stuck with the expensive option.
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Old 11-05-2020, 01:38 PM   #9
SightsSetStudios
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Bummer. It seems more like the oil though, correct? I don't see how everything would shift fine until about 30 minutes of highway driving and then start grinding mechanically. That seems more like the oil is thinning out.
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Old 11-05-2020, 01:55 PM   #10
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It's really impossible to tell which is the real culprit.
You are in trial and error territory.

Yes, warmer oil may be thinning out but remember:
1. You changed oil 6 months ago so, all things being equal, thinning oil would not be suspect at this time, generally speaking.
2. You are using 75W-90 weight. That means when the oil gets hot, it goes up from a 75 weight to a 90 weight, actually getting thicker.
3. Don't know where you bought your oil from but, like many things, there are counterfeit products everywhere being sold in OEM-looking packaging. So, depending on where you purchased your oil, maybe it's just some cheap Chinese knock-off.

Are you the original owner?
Know anything about previous owner(s) and how this car was driven?

Also possible that if this transmission was really beat on, you are the one seeing the results at a much later time than when it may have been abused.

You can drive 5MTs hard, but a bug difference between driving it hard and knowing how to drive and just outright abuse.

As Clint said though, Motul is a very good product so I would not hesitate to go with Motul either. Used that in my 6MT with out issues as well.
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Old 11-05-2020, 03:19 PM   #11
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It's an 01 with 200k miles on what I believe is the original tranny. I definitely think that it would at least have some wear. I bought the oil at Napa so I trust it's legit, but maybe it would be worth giving the better stuff a shot.
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Old 11-20-2020, 06:55 PM   #12
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By 200k miles your syncros have begun to physically wear. At this point, you should plan on using more expensive fluids to improve shifting, until you decide to address the root problem of a worn out transmission.
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Old 11-22-2020, 10:51 PM   #13
martinjs
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Not subaru transmission experienced but after owning and driving many manual transmission cars over the years, only one has ever had significantly improved Syncro function after switching oils (1977 BMW 630) where the 2nd gear Syncro stopped grinding on downshifts with a synthetic gear oil. Known quirk of the car. Other than that, while I have noticed in general 'smoother' shifting with high quality synthetic oils in MT's including worn transmissions, if a Syncro is worn and not working properly, only replacing the Syncro will make it work and shift properly again. Maybe some improvement for a while with better oil, but, as previous posters say, ultimately gearbox needs rebuilding to get better shifting.
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Old 12-14-2020, 10:56 PM   #14
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Has Uncle Snotty's cocktail fallen out of favor?
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Old 12-15-2020, 10:18 AM   #15
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Quote:
Originally Posted by WRXRgr View Post
Has Uncle Snotty's cocktail fallen out of favor?
I presume you are joking.

" Uncle Scotty's Cocktail " was crap from day 1.
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Old 12-15-2020, 11:34 AM   #16
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the 5th gear in my STI 6 speed has been doing this for awhile now. It grinds every time I shift into 5th unless I rev match perfectly, and it seems to get worse when the oil gets warmer.

Ive tried Motul Gear 300, Amsoil Severe Gear 75w-90 and Subaru trans oil and none of these helped.

I'll be pulling the trans this summer to get rebuilt.
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