-Check rear cam seal(s), telescoping mirror helps.
-At timing belt/water pump service check front cam seals and crank seal
-On a recent timing belt service, I discovered my oil pump housing bolts were all a bit loose and there was a bit of oil seeping, but not enough to cause consumption.
-If you see smoke after extended periods of idle, long stoplight, or sitting parked for a few, then likely valve stem seals. Inexpensive parts, pesky labor, but well worth it in my opinion. I've done two of our trucks and the oil usage dropped to next to nothing. Yes the bottom ends on the 3.5L seem to be quite good if taken care of.
This repair can be done "on vehicle" with care. Air compressor and fitting to charge each cylinder to hold the valves in the closed position, (or the rope trick with cylinder "up"). Best with this spring removal tool and especially these valve stem pliers. I don't know how I got it done without the pliers the first time. Those old valve stem seals seemed welded on! (not really, just a pain).
Get your mind "right" and be patient. It is 24 times, disassemble, remove seal, clean, install seal. Except when you forget to remove the little clear plastic seal install guide and put the spring and keepers back on...Doh!
Lisle Valve Spring/keeper tool set
Schley valve stem seal removal tool
I think the valve stem smoke issue is one of the reasons so many folks give up and get rid of an otherwise perfectly good Montero. They assume oil rings, compression rings, or piston problems. Reportedly, the valve stem seals get cooked if there is an overheat issue, often that is related to the water pump o-ring or one of the coolant bypass pipe o-rings failing and allowing coolant to leak without major notice. I think the valve stem seals themselves had an upgrade in design somewhere along the line.
Even though it's a big job, I still recommend it, and think they will last a long time if the coolant system and oil levels and leaks are kept under control. I used Fel Pro seals on closeout from RockAuto, it may have cost more shipping than the parts themselves, and they looked to be fine quality.