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Synthetic lube bad for paper lining in synchros?

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wparks

10+ Year Contributor
99
55
Mar 19, 2012
Fort Collins, Colorado
Ok- here's a weird one-

Just had my 1G AWD manual transmission rebuilt by a very reputable local shop (Advanced Transmission and Clutch in Fort Collins, CO). They deal with many older and racing transmissions and have a stellar reputation.

I was asking the main guy about transmission oils, and all of the disagreements on the forums about amsoil vs redline, vs BGsynchroshift, etc etc. He said "look- the syncros in this car actually use paper material (he said lining), so stay away from anything synthetic as it will destroy them." He agreed that I should stay away from GL5 because it's corrosive to the yellow metal in the synchros. He said just use regular GL4, non synthetic, and you'll be fine.

Has anyone heard this? I thought synthetic (my favorite is Amsoil) was totally the way to go, and I have never heard of paper anything in synchros. You?
 
Early 1g transmissions used organic lined syncos, and yes, they don't mix well with certain oils. Later mitsu went to brass syncros, which are not sensitive to types of oil. I believe you can upgrade to the brass syncros, but I'm not 100% sure about that.

I run MT-90 and I have zero complaints so far, shifts smooth as butter at 7.5k
 
If it helps, I always use MT-90. In my personal car I run a stock 92 awd trans, and it has been fine (knock on wood). Ive also never heard of this, though, and I find it interesting.

Does it have a warranty? If so, Id use what they recommend to keep that warranty intact.
 
Yeah I heard of this for the 2g 420A FWD transmission... At first, I used a heavy generic 75W 90 GL-5 gear oil for a while and the shifting got rougher and rougher, so I swapped it out for a super thin ATF+4, yea thats automatic trans fluid and it worked really nice, smoothed out the shifts and supposedly good for the paper lined synchos. I think for the 4g63 transmissions you'd want to use anything GL-4, not sure about paper lined synchos in these ones.
 
I replaced the factory gearbox lube with BG SyncroShift II in 2002. I have not had any issues. This is however a very low mileage car. I've only put on 12k miles since then. This is a 1990 FWD car.
 
Use what the shop tells you to use. If not, you're basically throwing any warranty out the window.
 
That's a great link ErikTande! I love this part:


Transmission, ALL Mitsubishi:
DO NOT use Synchromesh, BG Synchroshift I or II, or Royal Purple. These fluids will destroy synchros and gears and void your warranty!

I have no idea where to get that specialized Mitsubishi factory oil- sounds like MT90 is where it's at.
 
I believe you can upgrade to the brass syncros, but I'm not 100% sure about that.

I don't know the a brass synchro is an upgrade. It certainly lasts better, but a new fiber synchro shifts better. I run old used fiber ones with gl5, and shift at 9500 with a 2600, and have good results.

Always run a heavy gear oil in these transmissions.
 
:confused: I guess the dealership might be a good spot to start? :confused:

Yes, Yes, I know Goodhart the dealer has it. What I was thinking when I wrote that is that I won't set foot in a dealership unless my life depended upon it. They don't even use Vaseline there!

This shop specializes in older transmissions, and the rebuild was an OEM style rebuild, so I don't mind that he used the original type of synchros. The last ones lasted 185K of hard high-school driving, and I'm not a street racer, so I'm happy. It's still wearing in so it's not as smooth yet as I like, but a few thousand miles of gentle care and some MT-90 and I think it will be as sweet as YooHoo. Next time I'll probably go to Jacks- Thanks for the recommend ErikTande.
 
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