Try running Castrol GTX RX SUPER 15-40. It's for diesels, so you can expect it to really clean the engine, including the rings.
Now this won't fix the ring problem completely, but it might help. Run the RX for the usual maintenance period or shorter if the oil starts to get very dirty. Keep using it until the oil starts to look reasonable between services, then you can switch to Mobil 1 15-50 synthetic, assuming the consumption has dropped, othewise stick with the RX or GTX 20-50 (non RX).
As far as I'm concerned I believe that Isuzu should be brought to justice for this situation, as they are very aware of the problem they have caused for Late model Rodeo and Trooper owners, so if anyone knows of a good product liability lawyer speak up and maybe we can start a class action law suit to collectively address these issues.
BTW I have a 1992 3.2 DOC Trooper, which doesn't burn a drop of oil between oil changes, however, at 140K miles it has started to show it's age, so I'm now looking for a later model. Now that being said, I sure as hell don't want to throw my money away on a Newer Trooper while the folks at Isuzu continue to ignore the problem, so I'm trying to find a good low mileage 1996-97 4x4 3.2 Trooper Limited or LS with DOC, no rust, heated seats and accident free, located somewhere along the Eastern seaboard.
As an added note, late model exhaust manifold heat shields (two bolt style) can rust around the mounting bolts leading to a premature failure, resulting in the shield becoming loose and getting lodged between the manifold and the brake lines on the passenger side and the manifold and steering column linkage on the drivers side, now that's gotta be considered an NTSB safety issue.