I have the GM Techline service manual CDs and the Isuzu NPR is on them.
I just checked and there is only one very minor TSB listed for the 2003 Isuzi NPR (I have no information later than 2003):
#02-07-30-025B Harsh Shifting, Delayed Upshifts with Possible CHECK TRANS Lamp Illuminated, Possible DTC 21 Set (Perform TPS Relearn Procedure) - (Jun 27, 2003)
Apparently it's got a 4L80E or a 4L85E automatic transmission which is made by GM.
Detailed information about this transmission is here:
http://media.gm.com/division/2005_prodinfo/powertrain/transmissions/05_auto_trans_specs.html#mt1 Noteworthy is the fact that it uses regular Dexron III transmission fluid. Many automakers are switching to proprietary fluids which cost more (like Ford's Mercon V)and are not available outside of the dealership (like DaimlerChrysler's ATF+4).
Since regular fluid changes are the best way to ensure long transmission life, the easy availability of the correct fluid should be considered.
