At what point should i change my front and rear dif. oil??

IMO I would do the 1st change around 15k. After that with synthetic maybe every 30k.

Do it at 1K--mine was filthy,funky color, and the magnet was loaded with metal dust on the rear.The front I would do as per GM's schedule unless you're off-road a lot.Then every 25K.

I'd do em both at 1K, you would be amazed at what comes out.

At what point should i change my front and rear dif. oil??
Good idea to change them early, the proof will be when you change the fluids, you will see how dirty the fluid is , and the magnets will be covered with crud, probably minute metal shavings. Cost of fluids will be minimal. Most members change fluids between 500 miles to 2000 miles, and refill with synthetic fluids.

I seem to be the exception to the rule here. I just changed mine @ 1600mi, and can honestly say it was a waste of money. The fluids were fine, and there was hardly any metal sludge on the magnetic plugs at all.

I'll check mine again @ 5000mi, but will not blindly drain them again expecting to see the worst.

What gear oil do ya'll recommend and how much fluid does it take??

Mine took 3 liters (3.18 U.S. quarts). I decided to stick with the GM synthetic, known here as "grape juice". It's expensive, and I have no reason to believe that it's better than Mobil 1 or any of the other fluids recommended on this site, but I ultimately decided that I didn't want to worry about any potential warranty issues down the road.

There seems to be a significant variance between fill levels reported on this site; ranging anywhere from 3 to 4 quarts. I suspect that those who are requiring 4 quarts may not be letting the vehicle sit long enough after a drive before draining the fluid. I read that it should sit at least 1/2 hour on level ground in order for fluid to drain back into the diff from the axles. I followed this guideline and ended up filling it with less than 4 quarts.

Any special tools to put the oil in the rear end ie pump or just squeeze the bottle.

Any special tools to put the oil in the rear end ie pump or just squeeze the bottle.

I used a funnel designed for differentials. It is basically just a flexible hose about 18 inches long with a cap designed to screw right on to the oil container. It shouldn't cost you more than about $4 at your local auto parts store.

A pump would work, but it would be slower than the funnel.

Whether or not you could effectively sqeeze it in would really depend upon the size of the bottle. The fill plug sits about midway between the top & bottom of the diff cover, at the rear. With the spare tire in place, it would be quite difficult to tilt an average size bottle above a horizontal position. I didn't bother removing the spare, but I suspect that doing so would give you enough room to do the job without the hose funnel.

I used a 1/4 clear plastic tubing and I stuck it over the end of the nozzle on the Mobil 1 bottle. I just squeezed it in there that away. Worked easy enough. Do you fill it to the bottom of the hole, or a half inch below it. Book says .5 to .7 of an inch from the bottom of the fill hole. I always thought when it began to run out, it was full. Curious what you did?

I used a 1/4 clear plastic tubing and I stuck it over the end of the nozzle on the Mobil 1 bottle. I just squeezed it in there that away. Worked easy enough. Do you fill it to the bottom of the hole, or a half inch below it. Book says .5 to .7 of an inch from the bottom of the fill hole. I always thought when it began to run out, it was full. Curious what you did?

I dumped 3 full liters in (3.18 quarts), which brought it up to 1/2" below the fill hole.

I've read here on the forum that it's pretty simple to change the Fluid for the rear and front dif. because there's a drain plug and fill plug.....but what is the White / black breather cap????????

I've read here on the forum that it's pretty simple to change the Fluid for the rear and front dif. because there's a drain plug and fill plug.....but what is the White / black breather cap????????
Above the drain and fill plugs on the side of the front diff., you will see a round disc around 2 inchs in diameter with a little black hose coming out of the center and going upwards. It will either be black or white in color. New white one around $16.75 from dealer. White is the new replacement part. Part # is 12479390.

Do those breather caps have to be changed when you but the fluid?

Above the drain and fill plugs on the side of the front diff., you will see a round disc around 2 inchs in diameter with a little black hose coming out of the center and going upwards. It will either be black or white in color. New white one around $16.75 from dealer. White is the new replacement part. Part # is 12479390.
I changed my cap just in case. Just did both at the same time, so there is no problem. Some people claim they did not have to change their cap. Supposedly, the black cap is not capatible with synthetic fluid.

there needs to be an how to article on this subject. How is going to do it?

So does anyone have any step by step directions??

So does anyone have any step by step directions??
The front is easy to do too. Easy if you remove the skid plate. Two plugs on drivers side of front differ. Bottom drain plug is also a magnet . Chances are the magnet will be very dirty. Vent cap is above the plugs. Need to change also if you put synthetic fluid in. Tighten to 18 ft. lbs. 15/16 deep well socket will fit. Takes two quarts of fluid. Very easy to do...

Sounds like you should change at 1k miles by the sounds of it....Looks like the break in has happened by that time. I didn't keep my 05 long enough to even get the miles on it to make the 2nd change but did the first one at 5k looks like the 07 I will do at 1k does anyone know when you should swap the tranny fluid completely out? I like to run synthetic in mine preferrably Royal Purple. I just want everything to get broke in before I go to synthetic oils as I've heard by not letting it get broke in then going to synthetic it'll never seat right.

What about these white caps? I have done everything else on my truck by using the do it yourself articles. Does anyone have a step by step on changing the front and rear diff? Throughly explain the white caps stuff too.

2001 Duramax CC 4WD Allison Tranny, Dually

What about these white caps? I have done everything else on my truck by using the do it yourself articles. Does anyone have a step by step on changing the front and rear diff? Throughly explain the white caps stuff too.

2001 Duramax CC 4WD Allison Tranny, Dually
Read reply # 19 for the front, the rear drain plug is on the bottom. Takes a 3/8th ratchet with a short extension to remove the plug. Fill plug is on the cover, takes 3 3/4 quarts of synthetic fluid, or 1/2 to 3/4th of a inch below the fill plug. You can put a short hose on the bottle tip and tilt the bottle up and into the fill hole. Very, very simple and easy to do.

If you take the spare down you can just squeeze the bottle in while sitting under the truck. VERY simple. Front needed a piece of tubing to work. I went Mobil 1 front and rear. 2 F & 4 R level with the plug in both cases. Worth the time & $ for peace of mind IMHO.

The front is easy to do too. Easy if you remove the skid plate. Two plugs on drivers side of front differ. Bottom drain plug is also a magnet . Chances are the magnet will be very dirty. Vent cap is above the plugs. Need to change also if you put synthetic fluid in. Tighten to 18 ft. lbs. 15/16 deep well socket will fit. Takes two quarts of fluid. Very easy to do...


Ok Im not using synthetic in the front end, I am using synthetic in the rear end. How do you take off the skid plate i havent looked under there yet so i am assuming it is simple.

I don't know how you use your truck, but many 4x4's around here never even see a dirt road, much less mud. Therefore, maybe an initial change on the front diff., but that should do it...IMO.

Ok Im not using synthetic in the front end, I am using synthetic in the rear end. How do you take off the skid plate i havent looked under there yet so i am assuming it is simple.
My plate had 4 #15 metric bolts holding it on. Was pretty simple to remove, just lying on back, looking up and holding and balancing plate, which is aluminum, and removing the bolts.

I don't know how you use your truck, but many 4x4's around here never even see a dirt road, much less mud. Therefore, maybe an initial change on the front diff., but that should do it...IMO.


I get after it on the off road. Not really mudding or anything, but when taking hay and such out it could be pretty muddy at times

I get after it on the off road. Not really mudding or anything, but when taking hay and such out it could be pretty muddy at times
Did you take off your plate? I just left mine off for now, cause it makes my oil changes a lot easier and cleaner, front diff. and motor. Not many hazards where I travel that I need my skid plate, as of now...