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Ok, I know the correct thing to do would be to replace all the seals and gaskets,... What if a person (me) were to fill up the rear ends, transfor case, with grease? We have on old d6d on the farm, years ago the planetary/finaldrive was going out, I filled it up with grease, I have put a couple hundred hours on it. still pushing right along. I know it isnt smart, what might happen? I dont know what is going on inside the final drive on the d6, but i know it doesnt leak and works fine..

In the winter it may not want to move and when it does the gas consumption will be way up there.

Several issues come into play here as to why that is not a good idea. The most important is that grease will not allow for proper lubrication of the taper roller beaings and gears in the axle differential & trans case because of the design. You would have to literally pack the housing so full of grease you would far surpass the cost of seals and speedy sleeves to just fix it correctly. And then it would not be right.
Case in point the front pinion bearing in the differential, it is designed to get an oil bath. How would you keep adequate grease in the pinion bearing alone? The thicker grease would also play havoc with the differential bearings at highway speeds due to heat buildup and friction, it would run most hot. There is a big difference between gear teeth meshing with oil vs. grease. Unless you never plan to take your truck out of low 1st gear, bad idea.
To put it simply, look at it hydraulically, with oil, the axles and gears work easily with out excessive heat buildup. Imagine you are running in 3 inches of water on a beach, now try to run and do the same work at the same speed in 6 feet of water, which is easier?

Montanaorbust wrote:Ok, I know the correct thing to do would be to replace all the seals and gaskets,... What if a person (me) were to fill up the rear ends, transfor case, with grease? We have on old d6d on the farm, years ago the planetary/finaldrive was going out, I filled it up with grease, I have put a couple hundred hours on it. still pushing right along. I know it isnt smart, what might happen? I dont know what is going on inside the final drive on the d6, but i know it doesnt leak and works fine..

Well, if you had ask me how long a D6D would run with grease in the final drive, I'd have said less than 200 hours for sure. Being very familiar with heavy equipment, and operating equipment much older than the "D" series of the Cat D6 tractor, it actually doesn't seem that long since the D6D was a front line machine, although as I think back, it's been around 34 years since the "D" series was introduced. I would venture to say you would learn quickly what is going on in that final drive gear box if you tore it down, you would surely see why it isn't good to run grease instead of the recommended 30W oil. Now that I've told my age, I can concentrate on the question. In your trucks components, lubricant FLOW is critical. Simply put, anyone can easily understand that grease won't flow as a liquid lubricant would. Starvation from lubrication would trash said components in short order. That point being made, it's obviuos you shouldn't do the grease thing in a diff or t/case, however if your decision is to fill her up with grease and give it a whirl; document the results well. I for one would be interested in just how long it would be until catastrophic failure occured. Keep the pin and paper close at hand, I don't think it will be long until you will need to be documenting.

Grease in any sort of gearbox not designed for it that has more than a small and very slow amount of RPM will cause a faliure, gearboxes designed for grease are usually marked as such. I have a "Groil" misture in my steering box but it is purpose mixed for the job, if I put the same material in a regular diff, gearbox or winch it would definently cause a very premature faliure, straight grease would cause a faliure even quicker...........

Regards - MM

So let me get this right, i should use a grease with poly.. Jk.. it was just a thought. It might be something i would consider in the field, if i had too, or if i had a rearend on standby, and was conducting a science experiment, But it would be interesting to see how long it would work before it failed. hmmm... men, thanks again for the feedback.

As for the d6d i suppose i never is ran it long enough to get hot, real hot.. its only use to clip some roads, drag a tree out of the field, push a dead cow, real little duty stuff. may be the only reason it still works.