I got the front differential oil changed today. Although I removed the plastic cover, it still gave me hard time to put the fill plug back on. I eventually figured it out that I needed two hands working together from two directions to screw the plug in. Can't believe that the oil looks so ugly for only 40K miles (see the attached photo). Seems needing to change it more frequently. Thank you for your help.

Color doesn't mean much. Some oils are dark like that from the bottle, depending on what kind of EP additives they use.
I'd still do the rear diff and the tc every 40k, those are the stressed parts, especially the tc since the clutch in there does controlled slip to vary front torque.

Nice work getting the fluid changed. The front differential probably turns fluid darker than the rear differential because it gets much hotter (it's bolted to the engine and sits right under the header). I changed mine for the first time at 80k miles, and then again at 110k miles; in the 30k miles between changes, the fluid still came out very dark... it probably cleaned up a lot of what was left behind from the first change at 80k, too.

The fluid felt slimy and appeared to contain a lot of very fine particles. I think changing it every 30K miles is the way to go. Sounds like the fluid cannot sustain heat attack. Wonder if its performance could be altered (or reduced) by the constant heat treatment as well.

But, but, it is life time fluid!!!!
;):popcorn:

Well, that's a good wish, but the reality is that the gear oil degenerates faster than you think. After changing the front differential, one symptom, "engine brake", has disappeared, which is good.

Thanks for the suggestion. Will do the rear differential next week. A new TC was installed late last year so no need to change fluid. Have you done trans oil change? I have a GA8HP45Z transmission and it seems the procedure is complicated. Have to use ISTA to adapt and balance the new fluid during and after filling. Any comments?

I did the rear differential oil change this weekend, and it was a piece of cake. The oil color did not look much better than that from the front differential, which tells me that both should be changed every 30K miles at least. Have you changed the transmission oil? That will be the next on my to-do list but I will wait until I hit 50K miles, 10K to go.

Yes I have. It’s easy. Make sure you get a new transmission pan/filter when you do it (get the ZF one). You’ll want to do 2 pan drops and fills. There’s plenty of threads here and elsewhere for a full DIY. Most important thing is fluid choice and making sure the transmission is at the correct temperature when setting fluid level.

Why did you do two drops and fills? to clean out the old oil from the torque converter? Many youtube videos only showed doing it once.