THANKS to all of you who helped by your posts. After breaking in the engine using dino oil changed at the dealer, I tackled the switch to synthetic at home. What a PLEASANT surprise. As suggested, I removed the air intake hose -- the 'S' shaped section from the turbo to the filter housing, a two minute job -- and the filter was easily accessed; just unscrewed it from the top by leaning over the fender and lifted it out. The oil pan drain plug has a simple a 3/8" recess that is SO much easier to work with than the usual hex head. I got the 15 quart pan (a little scary cause it looks like it's going to overflow) and Mobil 1 5W-40 Truck and SUV 5 quart bottles from WalMart and EASY! (Rather than try to carefully dump in 3 quarts from the full third bottle over the engine, I dumped out 2 quarts into one of the empty bottles on my bench where it was easy to see, then just emptied the remaining 3 quarts into the engine.)
ALSO, as was suggested here, I requested the xxxx04 reflash at the dealer and at 30,000 miles my mileage, precisely hand calculated, is up a solid 2 mpg to 18-19 combined city/hiway driving and a solid 10 mpg towing a 32ft 5ver in the ups and downs of Colorado. [I have a '04.5 325/600 1T shortbed automatic, stock tires] It may not be as good as some of you have posted, but I am pleased.
I LOVE THIS TRUCK! My first diesel.

 

Is Mobil 1 5W-40 Truck and SUV rated for our engine?

 

It is "Fully Synthetic","For gasoline and diesel engines" but most importantly rated API CI-4
Ken

 

The way I read his post he used Mobil 1 5w-40. The rest of the name "Truck and SUV " leads me to believe he used this product.

Here

It's CI-4 rated, so it should be ok for his truck as far as I can tell.

 

Cool. I haven't switched to synthetic yet, I will concider this one as I used Mobil 1 in my gassers before I got the diesel.