Well, I decided to drain the rear differential fluid on my truck which only has 5,600 miles. Wow, when I drained it, I was VERY surprised as the fluid was nasty looking. It had A LOT of gray particles and you could see the metal shavings in the fluid. The magnet also had a lot of metal attached to it. I guess the break-in period causes this but the stuff needed to be changed. The wear on the gear looked normal and even.

For those who are waiting to drain it at 15K, I would say DRAIN IT NOW . Unless you want that nasty fluid floating around in your differential, spend the $50.00 and change it.

1 - It used a little under 4 quarts of Mobil 1 Synthetic 75W-90 fluid.
2 - The factory had it filled to the fill hole. Once I opened the fill hole, some fluid came out.
3- I filled it to the point where it was barely below the fill hole, just about to flow out, just like the factory filled it.
4 - I put some MEDIUM (blue) Loctite thread-locker on the bolts to keep them from backing out due to all the bouncing/vibrations. The FACTORY had RED LocTite on them as it was a PIA to loosen them. I could see the red substance on the bolts when I took them off.
5 - It cost me $40.00 in fluid and then a couple cans of brake clean, for a total of $50.00. Money well spent.

 

Pet,

Your experience is the same as mine...except I went 15k on the first change interval. The original factory stuff was very nasty looking. Since then I changed out my diffs at 30k and 50k and the Mobil 1 75W90 looked like brand new. I felt like I wasting good fluid changing it after the initial change.

 

Can you get the gasket at NAPA or Oreilly's for that?

 

the gasket on my 03 was reusable

 

Just be careful with the bolts, people have had them snap off quite easily...

 

The gasket on 03-07 CTD's w 11.5" AAM Axle are reuseable.

 

I caught "heck" when I changed mine at 15K, I took it to my friends shop and told him I needed to change out the gear oil. He said at 15k miles, I said yes the book calls for that interval and from what I been reading some are changing at earlier mileage and finding it quite nasty. Popped the pumkin off and the oil looked brand new. Changed it again at 45K same thing fluid looked new. Funny how some are so nasty and others are not.

Tony

 

Do you use any kind of sealant when putting that gasket back in??

 

No , just wipe off the rubber gasket and clean any grit off the outter pan and diff bolt hole area .

 

I changed mine the first time at 30,000, it was almost as clean as the new stuff I poured it.

 

I changed mine before 10K and it was a nasty looking grey color, very dirty...Same experience with two new chevys I had owned in that past. I would recommend to anyone w/ a new truck to change the dif with the first oil change, and should be good with a regular change interval after that. Besides these things can come low from the factory.

 

Now, reading the replies, it appears that either...

A - The original fluid was NASTY looking after only 5K
B - The original fluid was OK looking after 30K

Why?? Well, this is what I believe. The gears MUST form a wear pattern and the ring & pinion are MATED FOR LIFE. So, once they form a pattern, they must be kept together or replaced as a complete set. That is a FACT.

So, why the nasty metal in the oil, some differentials have it and then other do not have it?

I believe the mating and pattern forming process in those differentials that are NOT getting the nasty fluid are NOT seating as they should. Those that do seat properly will get the nasty fluid. Once it is changed, the fluid remains like new even after 50K+. Why??? Simple. The nasty metal particles are only found when the initial break-in and wear pattern process is completed. Then only small amounts will be seen, even after 30K+ on the same fluid.

Those with the nasty fluid after 5K is due to the ring & pinion mating like they should and the by-product of this is the grayish metal particles. You have a metal on metal gears moving 7000+LBS of truck. They live a hard life.

Those that don't. Well, IMHO I believe that the ring and pinion was not set properly and they are not engaging completely like they should. I know back in my Hot Rod days, that guys WANTED the nasty stuff, as they knew the R & P backlash was set correctly. If they didn't get it, then it meant the R & P needed to be set again, by shimming. The wear pattern on the gears can also tell you if they are engaging correctly.

 

Pet05, that sounds like a good theory and all, but the stuff that came out of my diff was way beyond anything that came off the ring and pinion. There were a few flakes of cast metal, looked like they didn't clean the housing of all the casting crap rather than successful mating of r + p. Plus there was so much metal in the oil that the magnet on the fill plug looked like a mushroom.

 

The metal to metal contact and the initial mating of the R & P will cause the fluid to turn gray. Remember, the metal to metal contact of the two GRINDS all the particles into this finely crushed gray soup. Any large particles that make their way into the R & P get crushed. It is like a blender going round and round.

Once properly mated, then the only thing that can really "break" the gears would be metal fatigue, which is very rare as the metals are treated. I've only seen this happen in HIGH HP and TORQUE launches on the race track. My friends 1970 Olds busted the Spider Gear from launching 500 HP with slicks in a 3,800 LBS vehicle. This instant "hit" of high HP & torque can snap them. Sometimes the driveshaft u-joints are the "fusible link" and they break.

The point is that during the "mating" process, tiny potions of metal break off, this then gets grinded in the the R & P. Getting this metal out ASAP is the best thing. Once it is out. There is very minor wear unless it is being stressed. That is why those rears will last 200K-400K without a rebuild. The cleaner the fluid and the synthetic fluid helps A LOT.

 

I was looking at my rear one today, is it a metric bolts. It seemed like a metric 13 fit it, is this correct?