Gear oil wt

[kewltrks]

Hey all,
I believe in the manual it shows 75 / 90 gear oil correct me if i'm wrong, Question is if I doing medium to heavy hauling would you use a heavier gear oil like 85/140? cold to hot weather temp come into play as well?

Thanks inadvance

 

[KATOOM]

Differential fluid viscosity should be based on variables outside of what the vehicle manufacture considers "normal" service. Meaning, if you live a climate where its neither arctic cold or blistering hot then what the manufacture installed on the assembly line is fine. That said, if you tow often in the hotter climates and/or tend to tow heavy then it could be beneficial to run a heavier viscosity.
I use RP 75-140 as I sometimes tow heavy in triple digit summer weather, but it also doesn't get any colder around here than the low 30's upper 20's in the winter either. I wouldn't suggest running 85-140 as thats pretty thick goop. Either a 75-110 or a 75-140 would be better for what you're wanting.

 

[EricViviano]

75w-90 synthetic gear oil works well in all conditions and temps. Easy choice.

 

[AH64ID]

I called AAM years ago and asked about this.

Their response was that 75w-90 was ideal in all conditions, unless you were running above GCWR in Death Valley.

I don't think a 75w-140 is ever needed, but a 75w-110 might not hurt if you're always at/above GCWR.

 

[KATOOM]

I have a DANA axle and had this conversation with not only DANA but also three oil manufactures for their opinion and suggestions on differential fluid viscosity. Very interesting information.

Without getting all complicated, what I learned from them was 80-90 was fine for most everyday situations. Choosing a heavier viscosity is done based on the maximum running temperature (100*C) given the load and heat dissipation for the application. Vehicle manufactures base fluid suggestions/requirements on the ratings of their application, which is whats defined as "normal". In short, not all applications require a heavier viscosity and, for various reasons, not all applications would benefit from a heavier viscosity either. Example, a large class 8 differential can easily run a 80-90 and tow 80,000 lbs without overheating the differential. But if you pull around a 15k 5th wheel behind your Dodge during the hot summer heat at 65 mph and you'll likely cause the differential fluid temps to reach sizzling temperatures. Thats when the fluid degrades as it oxidizes and looses its lubrication properties and change intervals must be more frequent, and a heavier 75-140 could be beneficial.

That said, if you live where the winters can get sub zero then you'll want a lighter viscosity just to assure that the fluid has the ability to lubricate before getting up to running temperatures.

Lastly, all this is best understood if you have a differential temperature gauge, but not something most people bother with.

 

[kewltrks]

Thanks for the response, I will stick to what's recommended .

 

[AH64ID]

Just read your other thread. Medium to heavy towing with a Detroit locker will be, um... fun!

But if you pull around a 15k 5th wheel behind your Dodge during the hot summer heat at 65 mph and you'll likely cause the differential fluid temps to reach sizzling temperatures. Thats when the fluid degrades as it oxidizes and looses its lubrication properties and change intervals must be more frequent, and a heavier 75-140 could be beneficial.

With DANA axles Dodge does recommend 75w-140 for towing heavy, but that recommendation went away with AAM.

 

[RVTRKN]

Example, a large class 8 differential can easily run a 80-90 and tow 80,000 lbs without overheating the differential. But if you pull around a 15k 5th wheel behind your Dodge during the hot summer heat at 65 mph and you'll likely cause the differential fluid temps to reach sizzling temperatures. Thats when the fluid degrades as it oxidizes and looses its lubrication properties and change intervals must be more frequent, and a heavier 75-140 could be beneficial.

I stongly agree with this statement, and all TDR members should take this advise and concider a heavier weight oil and more frequent oil changes with trucks that have the 3:42 diff ratio pulling over the ratings. In all the bragging about how their 3:42 rear diff can handle high GCW, way above the ratings, my buddy is one of those owners, I wonder just how hot those diffs realy get. Class 8 trucks are properly geared and designed for 80K+ pounds, for the load they run on a 24hr a day duty cycle. The G56 comes to mind, the original design of a G56 in a medium duty was properly geared for the designed load. Even Mercedes thought the first G56 (G56 AD) equiped Rams needed a higher ratio due to the G56 design, changed to the lower ratio (G56 AE) because of the RPM's were way above popular opinion. My G56 runs very hot (G56 AE) and is why I designed my cooling system.

 

[AH64ID]

In all the bragging about how their 3:42 rear diff can handle high GCW, way above the ratings, my buddy is one of those owners, I wonder just how hot those diffs realy get.

Higher gearing should run cooler than lower gearing. But other variables such as input speed and torque will play a role in heat as well. So it does make you wonder how it all plays out on an average day.

 

[KATOOM]

Higher gear ratios always run hotter due to the increased hypoid action. As does also a heavier viscosity fluid due to the added friction and decrease in dissipation. Likely only about 10 degrees but nonetheless...
But again, it all comes down to the fluids ability to maintain maximum operating temperature within a given application. Just because 80-90 may run cooler than a 75-140 under normal every day driving, the 75-140 is likely to remain more stable under a load while operating at 100*C and not threaten higher temps. There's a reason racing applications run a heavier viscosity.
This is why I installed a temperature gauge on my differential. I wanted to know. Did it change anything?...no but now I have one more thing to watch.

 

[RVTRKN]

One other concideration in using higher vicosity oils, is that the heavier oil doesn't transfer the heat as easily as a lighter oil. So if towing heavy for many miles, it wont trasfer the heat as well and could actually be hotter than a lighter oil.

Racing, especially drag racing, with higher viscocity is not comparable, because it is changed more often and driven less miles at any given race. If drag racing, by the time you get down the track, it might have raised just a few degrees more than ambient. I know if I had a race car, the fluids would be my first and cheapest choice of maintaining it.

I run Mobil delvac 50 wt synthetic transmission fluid in my G56, and it would run hotter than the OEM ATF fluid, prior to the cooling setup I developed. It has a pump and filter and uses a OEM auto transmission heat exchanger.

 

[AH64ID]

One other concideration in using higher vicosity oils, is that the heavier oil doesn't transfer the heat as easily as a lighter oil. So if towing heavy for many miles, it wont trasfer the heat as well and could actually be hotter than a lighter oil.

Racing, especially drag racing, with higher viscocity is not comparable, because it is changed more often and driven less miles at any given race. If drag racing, by the time you get down the track, it might have raised just a few degrees more than ambient. I know if I had a race car, the fluids would be my first and cheapest choice of maintaining it.

I run Mobil delvac 50 wt synthetic transmission fluid in my G56, and it would run hotter than the OEM ATF fluid, prior to the cooling setup I developed. It has a pump and filter and uses a OEM auto transmission heat exchanger.

View attachment 113599 View attachment 113600

I think your cooling setup is pretty nice! But I often wonder how necessary it is for the G56.

The G56 is the longest running manual trans behind the Cummins, and has had more power and ratings put thru it than any other manual trans. It’s a workhorse, tried and proven.

I do think ATF is a horrible choice for a manual trans, but it’s hard to argue with the success of the G56.


That being said, don’t forget I did lots of things to my 05 to make it “better”... then again it did have 50% more torque than stock. Needless to say, the ‘18 will by and large remain stock... but it also has 95%, or more, of the modded 05’s torque.

 

[RVTRKN]

I've seen as high as 250* at just 15K GCW, this temperature was after the bearings were replaced for signs of overheating and 50wt oil. The G56 is not the best trans behind the cummins, although it has been the longest run of the manual. Back to oil, the cooling system is necessary due to it running hot and the fact it is running a heavy oil. The 50wt is needed to soften the NVH from the Cummins, that the SMF allows to transfer to the G56. BTW, why don't the other transmissions have this same issue? I believe its the design of the G56 and aluminum case that magnifies this.

 

[KATOOM]

I've seen as high as 250* at just 15K GCW, this temperature was after the bearings were replaced for signs of overheating and 50wt oil.

Just curious but what type of towing conditions caused this kinda heat?

 

[RVTRKN]

The most GCW my C&C (10.5K lbs) has seen is 23K, and at 35K miles I had the trans inspected. My clutch was toast due to a dealer ECM update glitch in 09 (Long story if you have the time.) and I contacted Richard Poels of Standard Transmission to have it inspected prior to the SMF conversion. To many stories of noisey transmissions once converted to a SMF. Richard sent me these pictures, and recommended replacing races and bearings.

I had the fast coolers and sensors installed when it was rebuilt at that time and had the sensors operational shortly after. The 250* temperature recorded, was in Palm Springs on the north bound side of I10 @ 95*. I was towing my boat in signature, which puts the GCW @ 15K lbs. This is why I delevolped my cooling system. The pic below is the 5ver I own, and with truck fully fueled, gear loaded and wet weight in the 5ver, is @ 23K pounds.

 

[RVTRKN]

I noticed the boat is no longer in signature, look at picture, it's light enough that the Tacoma can tow it and stay within the GCWR.

 

[KATOOM]

Can you tow that kinda weight in 6th gear? Or are you in 5th most of the time?

 

[RVTRKN]

I keep it in 5th gear climbing heavy, or any time the engine RPMs drop off. I will tow in 6th on flat runs when pulling my 5ver, as soon as I start an incline and RPMs start to drop, I down shift to 5th. Going over the Grapevine so bound, one of the steepest grades at a minimum of 6% for 15 miles, I’ve dropped to 4th and 2800 RPM.

 

[KATOOM]

Do you see a significant temperature difference running in 5th verses 6th, since OD tends to build a lot of heat.

 

[RVTRKN]

I was warned by Richard, not to run in sixth heavy, unless flat or downhill running. I actually have two sensors, 1ea in both fast coolers, and I can see a 5* difference from the two while climbing. The temps rise rather quickly. A buddy of mine thought they were out of calibration, until we stopped at a rest stop. During the idling, the temps synchronized together, and then we started the final climb of Baker Grade on the way to Lake Mead. The cooling system has one flaw, the heat exchanger goes between intercooler and radiator, and will pick up heat from the intercooler, under high boost.