Gary, the 3.5L engine was designed with extremely short piston skirts, and
insufficient quantities or size of holes in the oil scraper ring groove of
the pistons themselves. Apparently they did this to try to lighten the
piston, but incorrectly assessed the impact that this engineering change
would have on oil consumption. As the engine turns at speed, enough oil gets
past the piston oil control ring to get into the cumbustion chamber and
burn - that's why EVERY 3.5L Trooper you see has a black, sooty exhaust
pipe, instead of the clean, gray or light tan pipe you expect to see on a
gasoline engine.

I'm told that when Isuzu actually changed out engines that had run out of
oil, they put redesigned pistons in them that didn't allow so much oil to
get past the rings. But I'm also told that Isuzu has resisted doing a
wholesale recall on this and fixing everyones' the way they should be,
saying that oil consumption of 1 qt every 1000 miles is reasonable.

When I pulled a trailer from Seattle to Florida, I got 300 miles to the
quart. To me, that's unreasonable. But I could live with my Trooper if it
werent for the lousy gas mileage, which Nobody can explain to me yet -
especially when friends with 2WD Suburbans and Yukon XLs are getting 18 mpg
and I'm getting 16 at best, 12 when towing.

Hope this helps.
best,
steve

I've never gotten a good answer either. Strange to say this is normal for
many Isuzu engines. One has to check the oil level often and add as
necessary, Change at the recommended interval usually 7500 miles.