1G What to use for manual transmission "flush"?

[XC92]

I rebuilt the manual trans on my '92 Talon TSi AWD a few years ago to fix some issues. The trans has run great since, but it had a minor oil leak, likely due to my not RTVing it properly when I put it back together. So a few days ago I dropped it and took it apart, to RTV it properly this time, removing the old RTV and cleaning the case up a bit, inside and out (mostly out, the inside was pristine).

However, some of that old RTV probably fell into the case. I tried to be very careful but it was all but unavoidable. We're not talking about a lot here. I'm not removing the input & intermediate shafts, which would have made cleanup easier, as this isn't a rebuild. Also, I work outside as I don't have a garage or shed, and because it's fall and windy, there are always leaves blowing, and little bits of that and dust fell into the inside of the bell housing.

Again, I've tried to remove as much as I can but some will remain. I even flushed it all with water and wiped it off and let it air dry, but outside, where, yes, there are leaves and dust blowing. It's kind of a catch-22, or something like it.

Once I was done putting the trans back together and reinstalled it in the car, I was going to put in fresh Mitsu 75W-85 gear oil, 3 bottles of which I already have. But, given the leaf and dirt situation, and the possibility that there are stilll a few droplets of water inside, what I'm thinking of doing is putting in some cheaper sacrificial oil, running it briefly, dumping that, then putting in the new oil. Probably overkill but this is in case some grit got in there, which can wear down gears and such over time.

So what can I use as sacrificial oil? The old oil, which was also Mitsu 75W-85 with a little Redline and only has a few thousand miles on it but probably has fine particles in suspension that might remain in the trans? Or some really cheap oil, and if so what kind? We're talking running the engine with the clutch engaged and the trans in neutral for a few minutes, maybe a few miles driven, tops, so it doesn't have to have fancy additives (although detergents that won't harm the synchros might be nice).

Would cheap non-synth motor oil work, the thinner the better so nearly all of it is removed when I drain it so it doesn't contaminate the Mitsu oil? If so, what weight? Would a little additive help, like ATF, Seafoam or MMO? I'm probably overdoing it but I just want to get any junk that's in there out before putting in the good stuff.

...Ok, I found these two that should do the trick, pretty cheap at ~$5.45 each per quart:

Walmart Super Tech Super Tech 80W-90 Gear Oil $5.44
Walmart Super Tech ST GEAR 1QT 85W140 $5.42

Which would do a better job of flushing out the junk, the 80W-90 or the 85W140? And would 2 quarts be ok for a quick flush & dump, even though the trans requires 2.3 quarts? Maybe top off the last 0.3 quarts with an additive?

Btw both are GL-5, which I understand is bad for trans synchros, but is it ok to run it for just a few minutes since it'll be flushed and replaced with Mitsu GL-4, or would any residual GL-5 oil be bad for the synchros?

 

[RamenPride]

Not sure how much it costs now, but I actually ran this in my transmission (mixed with synchromesh) for a number of years. Bought it at Wal-Mart. It's GL-4 and the only difference I could find vs normal gear oil is that it contains anti foaming additives. I only changed it out because I had some leftover Redline MT-90 from another car.
80W-90 Premium Gear Lube

On that same note, Pennzoil syncromesh is still readily available and cheap enough to use as "sacrificial" oil.

 

[steve]

I'd be tempted to use some light weight engine oil (or Pennzoil syncromesh, it was pretty thin) to splash around and drain out if you are going to run it quickly without any load? I'd want it to drain quickly and gear oil doesn't really do that.

Since you're trying to get any dirt out I'd overfill a little too.

 

[XC92]

I'd be tempted to use some light weight engine oil (or Pennzoil syncromesh, it was pretty thin) to splash around and drain out if you are going to run it quickly without any load? I'd want it to drain quickly and gear oil doesn't really do that.

Since you're trying to get any dirt out I'd overfill a little too.

Not sure how much it costs now, but I actually ran this in my transmission (mixed with synchromesh) for a number of years. Bought it at Wal-Mart. It's GL-4 and the only difference I could find vs normal gear oil is that it contains anti foaming additives. I only changed it out because I had some leftover Redline MT-90 from another car.
80W-90 Premium Gear Lube

On that same note, Pennzoil syncromesh is still readily available and cheap enough to use as "sacrificial" oil.

Sounds like the Pennzoil GL-4 Synchromesh is the way to go, but at around $11 at Autozone it's almost as expensive as the Mitsu. Would the Walmart $5.50 GL-5 be ok though since it'll be dumped, or could even a little residual GL-5 be bad for the synchros? Just trying to save some money here.

But if motor oil would work, which weight, and does it matter which brand? I'd like to avoid any additives that could damage the synchros or seals. Walmart's synth 0W20 is $5/qt, and since it's thin it should clear & drain better. 2 bottles would likely be enough and I could top off the rest with some 5W-20 I have for another car.

Nothing in synth NON-high mileage motor oil that could harm the synchros or seals?

And, not really possible to overfill a manual trans since the excess will flow out the fill hole. Although with the front raised I suppose that it's possible to slightly overfill it because the hole is slightly tilted up.

 

[CrackedDSM]

Sounds like the Pennzoil GL-4 Synchromesh is the way to go, but at around $11 at Autozone it's almost as expensive as the Mitsu. Would the Walmart $5.50 GL-5 be ok though since it'll be dumped, or could even a little residual GL-5 be bad for the synchros? Just trying to save some money here.

But if motor oil would work, which weight, and does it matter which brand? I'd like to avoid any additives that could damage the synchros or seals. Walmart's synth 0W20 is $5/qt, and since it's thin it should clear & drain better. 2 bottles would likely be enough and I could top off the rest with some 5W-20 I have for another car.

Nothing in synth NON-high mileage motor oil that could harm the synchros or seals?

And, not really possible to overfill a manual trans since the excess will flow out the fill hole. Although with the front raised I suppose that it's possible to slightly overfill it because the hole is slightly tilted up.

Pop off the breather on top of the trans. It’s what shep told people to do back in the day because he always recommended running extra in his transmissions.


Also: could you not just use some cheap auto tranny fluid? It’s got detergents in it and it’s thin af.

 

[XC92]

Pop off the breather on top of the trans. It’s what shep told people to do back in the day because he always recommended running extra in his transmissions.


Also: could you not just use some cheap auto tranny fluid? It’s got detergents in it and it’s thin af.

Does ATF have sulfur additives? Plus, at $20+, a bit pricey for this. I've been throwing way too much money at cars lately and need to cut back.

 

[BLACK'98DSM]

I'd probably use 10w-40 engine oil. It's a similar viscosity as 75w-90 gear oil. The temperatures are scaled differently but very close at transmission operating temp. Even though it's temporary, I wouldn't put 0w-20 or 5w-20 in it. I normally break in 5 speeds of all kinds with high zinc racing 20w-50 engine oil. It's a little thicker than standard gear oil and has a better anti wear additive package.

The cheapest ATF seems to be over $5/qt. Depending which blend, it usually has the viscosity of 5w-30 engine oil if scaled the same. 10w-40 engine oil goes for a little over $4/qt. A small difference, but if you can properly lubricate/protect the internals AND save money at the same time, the choice is clear.

 

[XC92]

I'd probably use 10w-40 engine oil. It's a similar viscosity as 75w-90 gear oil. The temperatures are scaled differently but very close at transmission operating temp. Even though it's temporary, I wouldn't put 0w-20 or 5w-20 in it. I normally break in 5 speeds of all kinds with high zinc racing 20w-50 engine oil. It's a little thicker than standard gear oil and has a better anti wear additive package.

The cheapest ATF seems to be over $5/qt. Depending which blend, it usually has the viscosity of 5w-30 engine oil if scaled the same. 10w-40 engine oil goes for a little over $4/qt. A small difference, but if you can properly lubricate the internals AND save money at the same time, the choice is clear.

Is this because the thinner oil might not do as good a job of adhering to and removing any crap that's still in there?

And any advantage of motor oil over ATF and vice-versa? I was just concerned that ATF might have stuff in it that's bad for a manual trans. Smells a bit sulfery.

 

[BLACK'98DSM]

Most Mercon/Dexron blends of ATF contain ~100PPM sulfur dioxide. While it isn't much, perhaps that's what you smell.

Generally, ATF won't hurt standard manual transmissions engineered after the late 80's/early 90's or so. Clearances became tighter and machine finishing greatly improved, so a less viscous fluid can be used. Most T series transmissions from Ford, Mopar and beyond will gladly accept ATF. I've used it in Mustangs, N/T DSM's and more. The gear sets and synchros are very accepting. Many times the ball bearings and needle bearings get a little noiser as a less viscous fluid won't mask vibration & friction noise quite as well.

I would use engine oil as a flush agent instead of ATF in your case though. If the transmission were designed to operate using 75w-90 gear oil - which is roughly equivalent to the 10w-40 engine oil viscosity rating, and ATF is roughly equivalent to the 5w-30 engine oil viscosity rating (sometimes as low as 0w-30 in modern synthetics), I would take the safe route and use the thicker engine oil to protect against short term wear. Would a few minutes of ATF really hurt the transmission? Likely not, but keep in mind that this fluid will inevitably be left behind. A thorough drain would take days, and still a few ounces of fluid will remain in recessed cavities and tight clearances. A similar viscosity engine oil will mix kindly with gear oil, ATF will separate in periods of stagnation.

 

[XC92]

Most Mercon/Dexron blends of ATF contain ~100PPM sulfur dioxide. While it isn't much, perhaps that's what you smell.

Generally, ATF won't hurt standard manual transmissions engineered after the late 80's/early 90's or so. Clearances became tighter and machine finishing greatly improved, so a less viscous fluid can be used. Most T series transmissions from Ford, Mopar and beyond will gladly accept ATF. I've used it in Mustangs, N/T DSM's and more. The gear sets and synchros are very accepting. Many times the ball bearings and needle bearings get a little noiser as a less viscous fluid won't mask vibration & friction noise quite as well.

I would use engine oil as a flush agent instead of ATF in your case though. If the transmission were designed to operate using 75w-90 gear oil - which is roughly equivalent to the 10w-40 engine oil viscosity rating, and ATF is roughly equivalent to the 5w-30 engine oil viscosity rating (sometimes as low as 0w-30 in modern synthetics), I would take the safe route and use the thicker engine oil to protect against short term wear. Would a few minutes of ATF really hurt the transmission? Likely not, but keep in mind that this fluid will inevitably be left behind. A thorough drain would take days, and still a few ounces of fluid will remain in recessed cavities and tight clearances. A similar viscosity engine oil will mix kindly with gear oil, ATF will separate in periods of stagnation.

Thanks. I'll probably go with cheap but decent 10W40, like Walmart's basic brand at $5/qt. It's a synth blend but I assume it would be ok for a few minutes. It's not the high mileage version which could mess up the seals.

Btw would 10W-30 be ok for this use, since the Mitsu oil is actually 75W-85? I have some on hand because it's what I use in the Talon and in another car.

 

[1990TSIAWDTALON]

Any of it will work. Only leave it in for a few minutes of transmission rotation to let it get sloshed around then drain it. It's what I would do, but really, it should be put together in a "clean" room to avoid all of this.
Pennzoil Syncromesh worked good in one of my transmissions when it didn't need special fluid and was in a stock car (and didn't have time to wait on Redline or Mitsu). Just my opinions. All the above are good, do it the cheapest way is what I'd do. Drain then fill with your choice of proper fluid.

 

[pauleyman]

Thanks. I'll probably go with cheap but decent 10W40, like Walmart's basic brand at $5/qt. It's a synth blend but I assume it would be ok for a few minutes. It's not the high mileage version which could mess up the seals.

Btw would 10W-30 be ok for this use, since the Mitsu oil is actually 75W-85? I have some on hand because it's what I use in the Talon and in another car.

You're not running any if this long enough to deal with viscosity enhancers etc. I think you're way overthinking this.

 

[XC92]

You're not running any if this long enough to deal with viscosity enhancers etc. I think you're way overthinking this.

Perhaps, but when in a position of ignorance, better to overthink than underthink. There's nothing wrong with asking questions when you're not sure and concerned that you might cause harm.

 

[XC92]

Any of it will work. Only leave it in for a few minutes of transmission rotation to let it get sloshed around then drain it. It's what I would do, but really, it should be put together in a "clean" room to avoid all of this.
Pennzoil Syncromesh worked good in one of my transmissions when it didn't need special fluid and was in a stock car (and didn't have time to wait on Redline or Mitsu). Just my opinions. All the above are good, do it the cheapest way is what I'd do. Drain then fill with your choice of proper fluid.

Not everyone has a garage or shed to work on such things. I wish I did but I have to work on the street. One plus is that I know half the people on my block by now. People keep asking me if I can fix their car and they'll pay me. I have to beg off because I'm not a mechanic and would never mess with the car of someone I didn't know well. I just direct them to a decent mechanic. I could have made thousands by now.

Anyway, yeah, sounds like I'm good. But I didn't just want to assume that I could put anything other than GL-4 in there. Sometimes something "seems" ok but actually isn't. Trying to think of an example but I can't, but I know they exist, dammit Jim!

 

[92AWDHX40]

MM I never thought about flushing it out, i just drained the old out and replaced it as i needed. I mean i never used any flushing in my engine other than changing the oil. Now you may see some shiny stuff in it but that's normal especially if it's a fresh rebuilt. Again, i wouldn't worry about it.

 

[XC92]

MM I never thought about flushing it out, i just drained the old out and replaced it as i needed. I mean i never used any flushing in my engine other than changing the oil. Now you may see some shiny stuff in it but that's normal especially if it's a fresh rebuilt. Again, i wouldn't worry about it.

Working outside in fall with leaves falling and wind blowing, stuff gets in there, so this is just a precaution. I might end up just waiting until there's no wind and using some sort of blower to get as much crud out as I can. It's dry by now so nothing should stick.

 

[jdxnc]

Run a fill of Walmart 80w90 through it for a few miles, dump it and fill with the oil of your choice. Done.

 

[XC92]

Run a fill of Walmart 80w90 through it for a few miles, dump it and fill with the oil of your choice. Done.

With all the conflicting advice I think I'll just pour in some heavy cream and make a ton of butter with all the churning. Pretty good lube and all-natural.

 

[CrackedDSM]

With all the conflicting advice I think I'll just pour in some heavy cream and make a ton of butter with all the churning. Pretty good lube and all-natural.

Good luck getting that shit out.



Would smell nice, tho.

 

[XC92]

Good luck getting that shit out.



Would smell nice, tho.

I'll just drive around with the drain plug off. Neighborhood cats will love it.

 

[CrackedDSM]

I'll just drive around with the drain plug off. Neighborhood cats will love it.

Damn the cats, I'd be chasing after you and beating stuff away with a stick. Butter is expensive.

 

[XC92]

Damn the cats, I'd be chasing after you and beating stuff away with a stick. Butter is expensive.

But you'll appreciate it on all the popcorn I'll be popping out of the exhaust.

 

[XC92]

So I ended up putting the "old" gear oil back in, which is only a few years old and has just a few thousand miles on it, and is mostly Mitsu 75W85 with a bit of Redline of the same weight to make up for the slow oil leak that was the reason for resealing the trans.

I had to top it off, though, at least a quart and maybe more, so the leak added up over time. I used some fresh Walmart 10W30 synth, and will drive it around a bit before dumping it again and replacing with fresh Mitsu 75W85, but not before confirming that there's no more leak.

Given that it's a mix of "old" gear oil and 10W30 motor oil in a roughly 2:1 ratio, is it safe to drive around for a few days and 30-50 miles, nothing too aggressive, before dumping it to replace with only the fresh stuff? Or am I risking the trans by doing that? It's just that I want to make sure that the leak is fixed before wasting fresh oil on it.

 

[biglady112]

As stated. You are way over thinking it. Engine oil, atf, cheap gear oil. Cycle it through, drive it long enough to get it hot, dump it and replace with your desired fluid.

I have run 8 seconds on Walmart gear oil. FYI. No reason this should have drug on for three weeks asking the same question and you doing what you want to do anyway.

 

[XC92]

As stated. You are way over thinking it. Engine oil, atf, cheap gear oil. Cycle it through, drive it long enough to get it hot, dump it and replace with your desired fluid.

I have run 8 seconds on Walmart gear oil. FYI. No reason this should have drug on for three weeks asking the same question and you doing what you want to do anyway.

No such thing as overthinking something you don't know enough about. Not doing so leads to way more problems than doing so.

And the point of running it for more than a few minutes and drive it around is to see if the leak is still there. I don't want to put in the good oil until I know that it's gone, and that will take driving it around a bit. But I want to make sure that this oil won't cause issues if run that long.

I'm not big on "good enough" solutions.