Something with a good amount of ZDDP, supposedly. I use whatever the fuck is on sale, found in the garage or is given to me. 204,000 '00 4.0 and is fine. Purolator or Wix filter.
Command-Trac or NP231:
The shift indicator pattern is 2WD > 4WD high range part-time > Neutral > 4WD low range part-time.
Select-Trac or NP242:
The shift indicator pattern is 2WD > 4 high range part-time > 4 high range full-time > Neutral > 4 low range part-time.