topless

Gear oil has additives specific to extreme loads between sliding metal surfaces. The one that tells you the load rating is the API-GL, it references all the others http://www.finalube.com/reference_ma...Categories.htm

Heat and shear are the 2 things that break down gear lubes. I prefer a 75w-90 with a GL-5 rating vs the 85w-140 because the only thing you're doing is generating more heat with the higher viscosity. Thicker takes robs more power and runs hotter. I found this out the hard way by burning up a rear end towing in 100 degree heat.

Deesil

Like state above (a couple of times) there is mechanical sheer that will break apart oils on the molecular level. It is more pronounced with gear mesh so gear drives on the front of motors can accelerate the loss of an oils ability to protect the motor. The big difference in syn oils is that syn oils have a shorter more robust molecular chain that is more difficult to break apart than a conv. oil's longer weaker chain. That is the main reason why syn oils alow a longer time between changes, they don't break apart as easily with sheer and heat. I still run dino oil but I'm fanatical about changing it every 3 k miles. I think it to be a better alternative than leaving syn oils in a motor longer with suspended debris staying in the motor longer, even tho the mechanical protection is about the same between the two alternatives if you only consider the oils molecular chain.

dbunhook

I understand the argument that oil breaks down due to shear but I don't know if I agree. If oil wears out from shear, why would we not see it loose its viscosity over time?

I also understand that oil can get contaminated with combustion byproducts and suspended metal particles and that is a good reason to change oil frequently.

But rear differentials are not the same as engines.

Why do automobiles and almost every light truck out there except Dodge, have rear differentials that are lubricated for life? In other words, why do we have to change our rear dif. oil every 15K when no other manufacturer recommends it?

$oC@l CTD

It was always explained to me that the "time" factor for changing oil doesn't come into play because as it sits, it breaks down, but more to protect the components that hadn't felt oil in a specified amount of time.

Example: I am in the military and deploy for 6-9 months at a time. Now, imagine my truck, on a worst case scenario, didn't get started for 9 months straight. Surely in that amount of time, ALL the oil that was coating the walls, components, etc. of the engine has ALL settled to the bottom of the pan, leaving everything in the top parts of the engine DRY. Everybody knows that when you heat metals up to high temperatures then leave them to the elements for any extensive amounts of time, that breakdown occurs fairly rapidly. So now you have to ask yourself, are there rusted/broken down parts inside your engine now due to a total lack of lubrication for such a long amount of time...in which case you would run the oil IN the engine for a while, get it hot, then change it all out.

Perhaps I am just talking out of my rear end...I'm not "oil-ologist" But it seems to make sense to me?

Deesil

Well I don't think anyone can intelligently comment on why Dodge policies/recommendations are different from any other manufacturer. Who knows. Dodge has some stupid policies and so does every other large corporation. Who knows, maybe some ring gear broke and locked up a rear end and killed someone on a highway and the attorneys blamed it on corporate change interval recommendations. Highly unlikely, but here in the good ole' USA nothing would surprise me when it comes to litigation.

While I think every a 15k interval is excessive, it wouldn't hurt anything. However, by no means does any other manufacturer make a differential that is lubed for life on the original gear oil. You leave the same diff oil in for 30 years you'll have a VERY loose rear end at the very best. Then again, how many of the cars built today will see much more than 15 years? Will any of them see 30? Not likely. The truth is, with the power and torque of today's diesel engines in a 9k lb truck, that rear end takes A LOT more abuse than a 55 chev diff or just about any passenger car will ever see.

The military supply trucks get lube changes like underware, mainly because a break down can cost someone their life. The average person just gets inconvenienced.

1sttruck

I've seen a used oil analysis judged to be 'good' by the labs and others, on what was later found to be a sludged engine. A UOA tells you the condition of the oil, not necessarily the condition of the engine, especially when you're buying a 'cheap' UOA.

The 15k diff oil change interval seems to be a wost case change interval, identified by engineering. Marketing gets involved and in order to make it simple for customers and to cover their warranty costs they force the change intervals into two categories; normal and severe service.

topless

When the hydro-carbon chain is broken, it allows an oxygen molecule to attach in place of a hydrogen and because oxygen has 2 attachment points instead of the 1 on hydrogen, the chain continues to lengthen, increasing the viscosity.
When oil breaks down, it always gets thicker, it only gets thinner if it's diluted with another liquid.

Deesil

Another byproduct of that process is that oil becomes more hydroscopic (sp?) with molecular breakdown does it not? (I'm really asking here because it's been awhile and my brain doesn't work like it used to)

CatDiesel_762

You sure sound like an "oil-ologist" You are some what correct about the engine being dry. However, I think the componet failure due to the engine or unit being put away hot is not the case.

I think you need to consider the reason is condensation. Parts like your engine, transmission, fuel tanks and differential will have componet failure due to rust. Which is caused by condensatiton inside the compoent. Why, because air is laden with moisture and air is pretty much everwhere here earth, even inside our engines and transmissions. Allowing the truck or vehicle to run until it reaches operating temp allows such moisture to evaporate from the oil and the oil to coat most of parts that need an oil bath. The oil bath will then remove the dust that has settled inside the engine or differential. Granted, the differential, transfer cases and manual transmission need to be driven in order for the oil to be spread through out the componet and heated up. This is beacuse these unit have no oil pumps to pump the fluid through out the componet. On another note, most engine do not reach operating temp while sitting idle, they need to be loaded down with a load. By driving the vehicle we create a small load on the vehicle, even when empty. Thus bringing the vehicle up to temperature.

I am curious do you ever pull a valve cover and coat the rockers top of the motor with oil prior to running your vehicle or truck after a prolong deployment?



Shawn

WhoozYaDaddy

Catdiesel:

Off the subject: changing oil does not increase oil consumption for our country. The "recycled oil" is burned and used for heat. If the recycled oil was not used another petroleum product would be used in its place. I am not arguing against an oil analysis but an analysis won't decrease foreign dependence on oil in the least.

CatDiesel_762

Actually, in theory it should. Because we will no longer be changing oil on the whim, but when the componet or unit oil supply has reached its life expectancy. This theory only would work if every one did this and went to longer drain intervals and then you would see a large reduction in demand for fresh oil. We are already seeing reduction in the large maintence cost on consumiables and down time, such as oil changes and cost of working machinery not working by using this method. Why do you think alot of companies are now going to longer drain intervals. A benefit which is a reduction of their operating cost on maintenance. The only flaw is then the nations economy.The oil and gas industry, the refinerys, and the local maintenance/repair industry employ a good chunk of the nations economy. Are we wiling to lose this source of income for so many just to reduce cost on maintence and to save our enviroment?

You are correct that another product would probably be put in its place. However, we are already reaching a climax point where we need to find other sources for energy and lubercation. One that can be constantly renewed with out much more added energy. Maybe we should try for a garbage incinerator to power our cars, like on the movie "Back to the Future Two. Since, the cost effectiveness of exploring and collecting an oil reserve is only going to continue to rise.

Shawn

$oC@l CTD

Yep, you picked up and finished the topic where my Ph.D in lubrication ended I've been fortunate with my deployments. My truck/car don't sit any longer than a week at a time before my wife either runs them until the needle is at 180* or just fluctuates between driving the two to work, so the scenario i decribed above wasn't aimed directly at me, just a theory on why "time" is a factor in changing out the engine oil. I could imagine the rockers looking PRETTY ugly after 9 months of just sitting

CatDiesel_762

What really funny is, with all this talk about oil and oil contamination, and I do not even listen to my own advice. I just went out and started my Lawn Mower that has been sitting for over six months with out draing the oil. I just checked it and pour fresh fuel into the tank and pulled the cord. It smoked and smoked some more. For a while I thought I was misquito smoking the neighborhood.

Shawn