I just purchased a used 2003 GMC 2500HD D/A with approx 46k mi and I usually like to change all the fluids when I purchase a used vehicle because I don't know when the previous owner last changed them. I have some general questions as follows:

1. Transmission:
a. I plan to change the filter and fluid. Can I use Mobil 1 synthetic?
b. Should I change the internal and spin on filters?
c. How much fluid will I need with the Allison tranny?
d. Should I flush the whole tranny, or just replace what's in the pan?

2. Differentials/Transfer Case:
a. Do they come with synthetic from the factory? I think the manual
says the rear has synthetic, but the front does not. What is the
recommended interval for them?
b. What type of fluid does the transfer case use? Manual lists a GM
spec fluid. Do I have to buy this at the dealership or is there an
alternative?

3. Fuel Filter: How often should I change it?

4. Engine Oil: The dealership just changed it, but when it comes time for
me to do it, can I use Mobil 1 synthetic 15w40, or is there a special
diesel synthetic that I need to use.

5. When do I need to change antifreeze? Is it 100k with the pink stuff like
it is with cars?

6. Air filter is fine.

7. When should the cabin air filter be changed?

Any other suggestions on things I forgot would be appreciated. I'm new to diesels, so forgive me if my questions sound stupid.

Look in the "do it yourself" section.

One of the members did an excellent write up on servicing these beasts.

OK, I scanned the do it yourself and still didn't find answers to most of my questions. Anyone out there care to help out a novice who wants to do the right things?

Welcome. My answers are below. Oh, and I am not a Tech, but figure they are tired of answering this stuff. The search function seems a little quirky at times, but most of this stuff is out there many times over in different threads. You might have to get a little creative in your choice of search criteria.

I just purchased a used 2003 GMC 2500HD D/A with approx 46k mi and I usually like to change all the fluids when I purchase a used vehicle because I don't know when the previous owner last changed them. I have some general questions as follows:

1. Transmission:
a. I plan to change the filter and fluid. Can I use Mobil 1 synthetic?
Any Dexron III fluid should be OK.
b. Should I change the internal and spin on filters?
Just the spin on. The internal only needs to be done at overhaul.
c. How much fluid will I need with the Allison tranny?
It should be around 7 qts with the stock pan. Add six and check it.
d. Should I flush the whole tranny, or just replace what's in the pan?
Just drain it unless you are switching to synthetic.

2. Differentials/Transfer Case:
a. Do they come with synthetic from the factory? I think the manual
says the rear has synthetic, but the front does not. What is the
recommended interval for them?
The front was not. I am planning on changing mine every 50K miles.
b. What type of fluid does the transfer case use? Manual lists a GM
spec fluid. Do I have to buy this at the dealership or is there an
alternative?
Transfer case uses Dexron III. Check the diesel supplement.

3. Fuel Filter: How often should I change it?

10K.

4. Engine Oil: The dealership just changed it, but when it comes time for
me to do it, can I use Mobil 1 synthetic 15w40, or is there a special
diesel synthetic that I need to use.
Should meet CI spec, but CH would be OK too. That is diesel specific oil that is OK to use in gassers. It doesn't work the other way. The only Mobil synthetic to use is Delvac 1 or Truck and SUV.

5. When do I need to change antifreeze? Is it 100k with the pink stuff like
it is with cars?
5 years and yeah, it's the pink stuff (Dexcool?).

6. Air filter is fine.

7. When should the cabin air filter be changed?
2003 and up don't have cabin air filters.

Any other suggestions on things I forgot would be appreciated. I'm new to diesels, so forgive me if my questions sound stupid.

Thanks. That helps alot.

Timberwolf, I recently bought my '03 pre-owned also, and did just what you are suggesting. I had the dealer perform a trans flush and sump filter change since my fluid was very grey/black. They couldn't get everything out, but it sure looks better. Set me back $140. Now I change the pan fluid and spin on filter every oil change - at least until I feel comfortable the fluid is clean enough. Then I'll go back to normal maint interval.

Also, get a fluid pump for the t-case. I don't think you can tip a bottle far enough to work (even with a flex-funnel). Be careful not to remove the plug from the bottom of the t-case (that actually is the shifter handle detent). Don't ask how I know this...

I went ahead and also changed the fluid in the front and rear diffs. Couldn't afford the GM Grape juice for the rear, so I got some amsoil synth instead. Kept dino oil in the front.

OBTW, don't freak like I did when my oil pressure dropped to 28 at a stoplight. That scared me too until I looked here and saw that everyone's did this. All part of getting used to a new/used truck.

Good luck. I love my truck!

Do not put dexron III into the transfer case! Use gm's transerfer case fluid. Dexron III will ruin it!. Check your owners manual.

Sorry, you are wrong. These are not AutoTrak transfer cases (those do use a special fluid).

http://www.dieselplace.com/forum/showthread.php?t=1181&highlight=Dexron+III

OK, I think I understand the confusion on the transfer case. A friend of mine has a Suburban with automatic 4WD which runs in 2WD and shifts to 4 only if needed. If I understand correctly, that's the one that needs the special fluid. The push button can use the same as the manual, Dextron III. Is my deduciton correct? Lock, thanks, I have a fluid pump, but that's good to know on the t-case plug. By the way, I know if you change front diff fluid that you have to change the vent cap from the black on to the white one, but why? Whats the difference?

You deduce correctly. As I recall, the push button and floor shift in the HDs operates the same way (electro-mechanical change).

Check your vent cap as it is probably white from the factory. Mine (an '03 also) was. Supposedly the black cap wasn't compatible with synthetic fluid, but some here have run sythetic with those caps.