I have a 91 that had the same problem. Had the valve guide seals done and the problem remained! Turned out in my case I needed to replace the heads. Once that was done all was fine. You might want to do a cost comparison between just the seals and doing the entire head job. At that many miles, it might be better insurance to do the whole job, especially if you hope to keep her going.

Does anyone know if there is a way to determine if the engine needs just the seals done or needs more extensive work?

1991 Montero LS, fading red paint
Front and rear limited slip differentials
95 SR Rear Springs, torsion bar crank, 137,650 miles
33x12.5 ProComp Mud Terrains, Bushwacker Fender flares
3 litre V6 with new heads