Thank you both for your input!
It sounds like, if your sending unit and gauge are working, you might have some corrosion or a short on the wiring for the warning lamp causing it to glow. Before you replaced the sending unit, when you say the warning lamp would come on, are you saying it would light up fully, day or night, more than just a faint glow?
Yes, it would be either completely on or off visible day or night before I changed the sending unit. Now it's really faint, and sometimes, though not often, not on at all. When giving it gas going uphill it often glows a little stronger, but still not near fully on.
there should be a oil pressure switch and a oil pressure sensor depending on your model Montero. The switch controls the light and the sensor is the sender for the gauge if equipped, it may be a combination switch/sender that does both jobs.
I believe* that the one for the LS does both. For the RS, the switch is the recommended part, and for the LS, the gauge is (and is considered a 'direct replacement'). The one I got is a little bigger than the original OEM one, but it was a little cheaper so I thought I'd try it. I was considering trying the more expensive one next, but wanted to see if people more knowledgeable than me could offer any other suggestions before doing that.
I will check the grounding when I get back. (Of course I start this thread the day I leave on a trip where I'll be gone for a month, haha!)
I will keep you all posted, and any other tips/suggestions are of course welcome as I'm very new to this but excited and eager to learn!