dbunhook

I just changed my differential fluid at 6K miles to Royal Purple 140 weight since I tow a 9K RV most of the time.

The owners manual calls for the differential fluid to be changed every 15k miles. Why do we need to change the oil so often? Does the oil wear out?

jamesbfishin

Dunno about it "wearing out" , but it can lose viscosity and when it becomes dirty from use it will not lube as well.

jsafe

dirty in the differential ?

jamesbfishin

Sure, but not dirty like engine oil. The gears are still going to wear a bit over time and the product of the wearing process will be suspended in the oil. Now changing it every 15000 miles is overkill.

Mr Bee

What happens to lesser oil is "Shear". That happens when run through gears and as the oil shears it has less lubricity. Happens to motorcycles that have a common oil supply for the engine and gearbox. The oil gets sheared and it does not work well in the engine. As fishin said, wear particles are suspended in the oil and can become and abrasive that is continously re-cycled.
One thing you can do is when you pull off the diff cover or install an aftermarket cover, using a good epoxy install several magnets and as the oil passes the magnets will remove the metalic particles that will wear.

KBennett

I am pretty sure Dodge already provided a magnet on the fill plug.

moterhead

Just happens that I can comment on this question. Happens that I had a uncle who had a PHd in Organic Chemistry. Spent his entire working career with Shell Oil. Had over 100 patents. I asked him that exact question. His reply was "Yes, it does. Oil is made up of long chains of molecules. with use, these chains will break apart due to mechanical shearing action, essentially wearing out". Exactly what Mr Bee mentioned in his post. This explanation makes me think the "miles or time" criteria for changing oil is not correct. That oil does not break down over time, only thru use. Heck, I never did pay any attention to the time criteria. Never even pay any attention to it. I just change it based on Manufacturer's recommended miles interval. NOT the recommendation of those in the business of selling oil changes. IMO, it's impossible for them to be objective. Just heard an O'Reilly AP ad extolling the virtues of 3,000 mile oil changes.

Bluedeviltorque

I agree with the statement of the oil breaking down at the molecular level, but I do believe that there is a time factor as well. As with any organic substance there is oxidation that will occur causing it to break down. Do I think this is cause for 3 month intervals with todays well refined oils? HECK NO!, but things like the snow blower, lawn mower and generator, I do change once a year because of this reason, even if only run for a handful of hours.

Justmyuneducatedtwocents,

Mike

CatDiesel_762

I was in the local parts store today and was looking at buying a few bottles to change my differential fluid. I noticed that on the back of 75W-90 bottles had more ASME, SAE, and MIL specifiction numbers for the approving the fluid. Where as the 75W-140 did not, but had generic qoutes saying "meets most of GM, Chysler and Ford service recomendations." I heard that GM's Heavy Dutys use the same axle, but use the 75w-140. I think I remember reading in the owners manual that Dodge does recomend the 75w-140 , but only in certain applications. So I decided to wait until I could look up the Dodge Service Specification and choose the correct fluid. That way, I do not have to worry about voiding the warranty by using an incompatible fluid that could cause premature failure.

I do think that the oil change interval are a little overkill, but then again. Most people over drive their vehicles. Back when I was working as a lube service technician at a local GM Dealership, I had a customer drive in with 15,000 miles on the factory oil in a brand new Camero. He then ask will his car's engine warranty be voided. This was over ten years ago when oil additives in the oil would allow the oil to be safely driven to 5,000 miles between filter changes with no additional additives added to the oil.

Shawn

Raspy

I have buddy that used to own a box truck that he never drove. It was just a shop that was parked permanently. The recommended oil change interval was (X) miles or 6 months. Well, he never drove it, but every 6 months he changed the oil. I used to ask him why and he just said it was because that's what was recommended by the factory. Oh well. Can't argue with that.

Wetspirit

CatDiesel_762

Have a oil analyist done on the oil. That way you can change the oil only when it needs to be done. Thus helping to save our enviroment and reduce our dependancy on foriegn oil. I really think this is the best way to go. That way you can take the time and mileage out of the equation deal with only the oils ability to perform.

Shawn

Raspy

That is the best way to see what contaminants are there and to see what's going on with the engine, but I don't see how that can say anything about the oil's long chain molecules breaking down. I don't even know how long that takes or if an engine can do it. The original question was "does the oil wear out?". Maybe it could be consideredd worn out if it was contaminated and maybe it could be considered worn out if it began to break down and had less film strength. How would we know if it had less film strength?


Wetspirit

CatDiesel_762

When looking at the oil analyist you should have a control sample. Any deviation in the used oil compared to the control will tell you if the oil is worn out. Most oils are created from a base crude stock and then addiives will be added. Resulting in your long chain molecules. These additives are what keep and help the base oil molecules remain stable during use. The loss of such additives will be your indicator of worn out oil. That is one reason why we are slightly able to recycle the base oil. Emphasies on slightly. Tho oil has to be re-refined using fresh crude stock and new additives.

Any engine or mechanical device that uses oil to lubericate, cool, and clean a working surface can break down the long chain molecules. This is partly why we have PVC valves and breathers on such devices. These valves/breathers allow us to vent the gases created by the chemical reaction of the additives breaking down or wearing out. Some times the carbon atoms and hydro-carbon atoms are displaced into the working surface. Thus a metals grain structure is altered by the oil. Hydralic Pump Blueing and Valve Varnish are examples of such displacement, granted these also needed heat to accomplish this feat. Then agian that why we use certain oils to cool such parts.

We could start an engineered debate about this subject. However, many factors have to be considered when taliking about used and worn out oil. That is why I will simply say get an oil analyist of the oil in question. If needed check for viscosity and ball shear wear. Use your control for the comparison.

Shawn

PoorMansCat

Dino engine oil has a relatively small heat range also. I've seen diesel engines get overheated, then once the problem is cured, the engine didn't seem to idle/accelerate right against a load. Changed the engine oil and the problem clears up. Granted, these were HUEI engines, which are much more depended on oil... Seen this happen on a 7.3 PowerStroke too..

jrussell

An analysis can tell you how much life the oil has left. Blackstone Labs does a "TBN" test that specifically checks for that.