Some 99-01 engines use oil, some don't. Some have reported success with keeping the piston oil rings (the culprit) clean with a high detergent oil such as Rotella synthetic. Keeping the EGR system clean is important, too.

In 02 I believe Isuzu fixed the ring design, engines with this newer design can be identified by the screw-in PCV valve, whereas the older style has a push-in valve. Perhaps your replacement engine has the newer design?

Steve Carlson - 95 Trooper LS expo rig
Serenity now!