Manual transmission shfting problems.

[HMan]

I am running Valvoline full synthetic 75W-90 Gl-5/MT1 gear oil in my 1994 Geo Prizm/Toyota Corolla with a 5 speed manual transmission and 1.6l engine. The gearbox shifts horribly with this oil, the only time it shifted even remotely smoothly was when I was near the top of a long grade on the interstate. It is like stirring peanut butter on cold days, and if I am not careful, I get pops and grinds. What gear oil should I use to replace this troublsome fluid?
http://www.valvoline.com/products/consumer-products/grease-gear-oil/gear-oil/59

 

[bmwtechguy]

The best shifting fluid that is compatible with your gearbox (that I have tried) is Amsoil's MTG, which is a 75W-90 gl4 manual gearbox oil. Redline makes a similar product called MT-90. Either is these would work well in that gearbox. GL-5 gear oils usually have wrong friction characteristics to work well with the synchronizers (too slippery), especially if there are any limited slip additives included in the formula. These additives are designed to work with posi-trac clutch units in hypoid differentials, not with gearbox synchronizers.

 

[salv]

Too slick for the synchros. Try Pennzoil Synchromesh. It is thinner than the OEM spec fluid, but it will do the job and improve shift quality. I run it in my Cherokee's AX-15 with great results. If you want synthetic, BG Synchroshift II is amazing.

 

[HMan]

I have almost bought the Pennzoil synchromesh fluid twice in the past 2 days, but I'm taking the wait and see approach because I'm worried about the oil being too thin.
Is there any evidence that it is safe to use the thinner fluid?

 

[bmwtechguy]

The Amsoil and Redline are both full synthetic and worth what they cost to me. This is not a fluid you change often and most units that size only hold 2 or 3 qts. Not sure I would go thinner than what Toyota recommends. If I'm not mistaken, they recommend a 75W-90 GL-4 or GL-5. I would take the GL-4 any day since both of the ones I mentioned are specifically designed, friction-wise, for manual transmissions and the proper operation of the synchronizers. The difference in shifting smoothness is night and day, in my experience. Some transmissions/transaxles(FWD) are spec'd for lighter fluids like the synchromesh. This is a lighter fluid and not GL-4 level of anti-wear additives, if I am not mistaken. These lighter viscosity fluids may work well, but I would stick with what is recommended in this application.

 

[salv]

My AX-15 calls for 80w90 GL-3. I have had no issues. If GM and Chrysler call for it in all of their current manual transmissions, from cars to trucks, I think you would be fine with it.
My transmission is an Aisan, same company that provides transmissions for Toyota.

 

[Hermann]

+1 on using a GL-4, I have Redline MT-90 in my old Beemer MC and it works great, Amsoil MTG would probably give equal results.

 

[Quest]

w/o knowing the operational/maintenance history of your car, all I could question is: are your synchro worn? If so, no MT gear oil gonna help resolve your shifting issues.

Good luck.

Q.

 

[bmwtechguy]

Agreed... if the synchros are shot already and the unit has been abused/neglected by previous owner, then no lube, no matter how good, is going to solve those problems.

 

[HMan]

It is the oil. Before I changed the oil the box would shift like a dream, it would shift perfectly, I even had a few friends comment about how smooth the box would shift.

 

[aquariuscsm]

Drain it asap and put Amsoil MTG in.

 

[ueberooo]

I've previously used a little tiny admix of ATF to thin down my 75w90. ATF has some nice properties. Not sure you'd want to risk this though.

 

[HMan]

If I have to buy a cheap oil, what would be recommended?

 

[HMan]

12346190 gear oil or transmission oil or transmission fluid replaced gm part # 12346074
Both of these can replace a GL-4 75W90 synthetic, and the Amsoil MTG is a close equivalent (read: better.)
I bought the MTG, I'm just posting this so that anyone searching Google for a oil for a Toyota corolla or Geo Prizm will have an easy lead on what to use.

 

[aquariuscsm]

Let us know how thw MTG works out.

 

[HMan]

Holy sweet tap dancing Jesus, this stuff is good. I can already downshift into fourth with ease on the highway, and I can coast twice as far as I could with Valvoline in the box.

 

[aquariuscsm]

The MTG is some GOOD stuff! The best imo.

 

[Raybo]

If Redline MT-90 doesn't fix it start looking around for a new one.Probably its eaten up the syncro's already if its that cranky.

 

[HMan]

Nope. Works fine now that I have MTG in it. I can barely remember the grinding, as such I'm amending my repair manuals to mention that the oil matters.