Thick vs thin gear oils/manual trannies.

[aquariuscsm]

I see people posting about using thicker gear oils in manual transmissions saying it will affect shifting quality. Do they mean the shifter itself will just be harder to move,or that it will cause grinding issues and interfere with synchro engagement?

Just curious because I see GL4 gear oils for manual transmissions in both 75W90 and 85W140 weights. I`ve thought about trying an 85W140 GL4 gear oil to maybe protect bearings better in this 100F weather we have here. What effect could one expect from this?

 

[Craig in Canada]

GL4s come a lot thinner than that. Usually they start around 7 cSt as ATF-like fluids (ie. RP Synchromax, Redline D4ATF) and go up from there. Redline MTL is a 70W80 if I remember correctly.

If your chosen MTF has proper friction modifiers included because it was designed for a manual and isn't just gear oil, motor oil etc..., you shouldn't have problems with grinding. Any difficulty would be from notchier shifting, particularly when colder. Of course 'cold' is relative in a place like south Texas, isn't it...

MT90 is already pretty thick. Is that the specified viscosity for your application or are you already bumped up a notch from MTL?

Craig.

 

[aquariuscsm]

I presently have MT90 in my transmission right now. It actually shifts the best when the transmission`s cold (first of the morning drives). Then as it gets good and hot,the shifting gets really notchy,almost feels like the gear oil`s too thin and not cushioning the shifts.

And just like you said,South Texas really never gets cold. Right now it`s been getting to 103F! Our winters are usually in the 70s.

 

[mechtech2]

Thicker oil is generally best for parts wear, and thin oil is better for shifting.
The thin oil is displaced easier by the synchros when shifting - it squeezes out easier.
Cars sure do differ in what they like in the transmission!
I don't think that you need a 140 in a transmission. You are not getting up to temperatures like an engine would. Clean oil of the original spec should be fine. Remember, thin oil cools better.

 

[HardbodyLoyalist]

Even among 75W-90 transmission fluids, you'll find substantial variation between viscosities. To wit:

Kinematic viscosity at 100C (cSt):

Redline MT-90: 15.6
Amsoil MTG: 13.8
Specialty Formulations MTL-R: 14.7
Motul Gear 300: 14.2
Royal Purple Max-Gear (GL-5): 21.0

 

[aquariuscsm]

Does that mean RP Gear Max is going to be the thickest (21.0)?

 

[Pablo]

Just want to clarify Amsoil MTG GL-4 75W-90 is 14.7 cSt @ 100°C.

PS Yes the RP 75W-90 is over spec thick 75W-90 GL-5. Not necessarily the bast thing for an MT.

 

[unDummy]

Yep, Maxgear is one of the thickest. But, I vaguely remember UOAs/VOAs of it being around 17cst. Either RP needs to update ALL their datasheets, or blend more accurately.

If your cold fluid level is correct and full, and you are having shift problems only when hot, you need to take some temp measurements.

If you're not happy with MT90, the Amsoil MTG or RP maxgear are worth trying. If you're loyal to Redline, 75w90NS or Shockproof Heavy should be considered. Other fluids like Motul gear 300 and Synlube, various OEM fills like Castrol or Ford honey, are options.

You should also inspect the rest of the drivetrain. Clutch fluid bleeding, hydraulics, mechanicals... should be inspected. Shifter bushings/linkages, motor transmission and drivetrain mounts can cause shifting issues. And, even tired u-joints could negatively affect shift quality. Clutch brand, type, and condition, pilot and throwout bearings, are also worth investigating.

My recommendation is to immediately top off the gear oil when cold on a level surface. If that doesn't help and you have more then 10k miles on the fluid, change it.

If you're simply looking for a thicker fluid for your weather conditions, research and consider a Mack TO-A plus fluid like:
http://www.imperialoil.com/Canada-Englis...ns_Fluid_50.pdf
http://www.macktrucks.com/assets/mack/approved_lubs/Mack_Trucks_Approved_TO-A_Plus_List_Rev1.pdf

 

[aquariuscsm]

Nah,I`m definitely not brand loyal to RL. I just tried it to band-aid my worn synchros in my transmission (as per advice on various Z forums). I have a new J-Spec transmission on order (hopefully it will get here quick). I may drain the RL and replace it with RP Max Gear if my transmission remains on back order any longer. The RL MT-90 just seems a bit too thin for the bearings.

 

[aquariuscsm]

Quote:

---

Yep, Maxgear is one of the thickest. But, I vaguely remember UOAs/VOAs of it being around 17cst. Either RP needs to update ALL their datasheets, or blend more accurately.

If your cold fluid level is correct and full, and you are having shift problems only when hot, you need to take some temp measurements.

If you're not happy with MT90, the Amsoil MTG or RP maxgear are worth trying. If you're loyal to Redline, 75w90NS or Shockproof Heavy should be considered. Other fluids like Motul gear 300 and Synlube, various OEM fills like Castrol or Ford honey, are options.

You should also inspect the rest of the drivetrain. Clutch fluid bleeding, hydraulics, mechanicals... should be inspected. Shifter bushings/linkages, motor transmission and drivetrain mounts can cause shifting issues. And, even tired u-joints could negatively affect shift quality. Clutch brand, type, and condition, pilot and throwout bearings, are also worth investigating.

My recommendation is to immediately top off the gear oil when cold on a level surface. If that doesn't help and you have more then 10k miles on the fluid, change it.

If you're simply looking for a thicker fluid for your weather conditions, research and consider a Mack TO-A plus fluid like:
http://www.imperialoil.com/Canada-Englis...ns_Fluid_50.pdf
http://www.macktrucks.com/assets/mack/approved_lubs/Mack_Trucks_Approved_TO-A_Plus_List_Rev1.pdf

---

Hey unDummy,where can I buy the Mobil Delvac 50? Do places like Autozone,etc carry it? I`ve never heard of it. RL MT-90 seems to thin out way too much in a hot transmission,especially here in South Texas when it`s been hitting 103F here.

 

[unDummy]

You need to visit truck repair shops, and any shops that specializes in heavy duty transmission repairs, along with truck parts suppliers.
Fluids are industry specific. Don't expect to find 18-wheeler gear oil in an autoparts store.

Delvac is just one of many. The mack list should be printed and you're bound to find one of the brands at one of the truck service areas.

 

[DOHCtor]

Personally, the recommended oil for my Mazda 626 chronos is 75w90 GL4\5 oil... But the car is 13 years old, has 183k miles on the clock and so far, the best oil i've putted in her was my very own mix of 1.7l 75w90 GL4 Motul Gear 300 with 1l of AC Delco Synchromesh oil... shifts a lot smoother with that... But when i filled her with dyno 80w90 GL5, it was very harsh and grinded a lot... But after, i've heard that like Nissans transmissions, those transmissions really don't like GL5 oil so go figure!!??

Didn't hardly made the transmission quieter or increased MPG for that matter...

 

[surfstar]

I have a 1993 Mazda MX6 2.0L that specs 75/80w90 GL5 oil. I've tried a mix of MT90/MTL 2:1 - was better than the old fluid, but not great (ran for ~60k miles). Had a second gear grind that was fixed by using Pennzoil Synchromesh - a MUCH thinner fluid; but the UOA came back with high oxidation and thicker visc (ran for ~25k miles). I'm now on 100% MT90, and just noticed this morning how smooth my shifting has been lately - still has some notchy-ness, but not bad and no grinding.

I'm at 225k miles now and with the original clutch. I'll probably leave the MT90 in for the rest of the cars life, unless I keep it over 275k.

I'm sticking with factory rec'd oil wt for gearboxes from now on. I'll just use a good syn (redline or amsoil) and not worry.

 

[gtx510]

Are you sure the factory service manual doesn't spec a 90wt GL-4? That's what the 6-7 Mazda FSM's I've seen say.

 

[surfstar]

Quote:

---

Are you sure the factory service manual doesn't spec a 90wt GL-4? That's what the 6-7 Mazda FSM's I've seen say.

---

I don't have the FSM, just the owners manual and that's what it called for in '93.