My wife drives a 97 Explorer with the 4.0 L SOHC that we got in 99 with 29K miles. We were the second owners, a funeral home had it first. It now has about 118K miles. I ran M1 10W-30 until 62K miles then switched to Castrol GTX 5W-30 because I began to notice a slight oil weepage around a few gaskets. The weeping went away almost immediately. The truck never has used any oil at all, to this day, and runs like a top. Oil changes are done at 3K intervals.
Changing the oil on this thing is a mixed blessing. It has enough ground clearance that I do not need to jack it up, but the oil stream coming out of the drainhole is directed at a frame crossmember and makes a mess. I bent a piece of sheetmetal into a channel about 12" long to intercept and direct the waste stream where I want it.
FWIW, I changed the front and rear diff lubes and transfer case lube to M1 products at 32K miles. Still in there with no problems.
This engine had a lot of newsgroup traffic about 3 or 4 years ago because of a problem with the cam chain tensioner mechanism. As I recall, it is a hydraulically actuated device similar to a hydraulic lifter in operation. When it is not functioning properly there is a tremendous clatter at startup. Ford denied the problem for a long time, then finally consented and did a recall. Make sure yours has been upgraded to the newest design.
Change your shocks if they have not been done yet. The difference is remarkable. If it is a 4 door, there is a fifth shock mounted transversly to the rear axle as well. You can do this one too for another $25 or so.
When you change plugs do it through the wheel wells and it is easy. Remove the tire assembly and the rubber inner fenderwell flap and the plugs are right there in front of you.
This has been a good vehicle for us, but I would like it more if it got better gas mileage. It routinely gets between 16-18 MPG with her driving, and up to 20 with me driving.
Good luck