I too have a '98 Ranger w/4cyl (but A/T) and have run ST Synthetic (sorry, no UOA). My truck ran fine with it (no explosions, no rainbows coming out the tailpipe, etc.), but I don't think the value is there now that ST Synth is $13+.
Now that my Ranger is the low man on the totem pole, it gets $.49 Chevron Supreme (5w30) and $2 Bosch filters. Truck runs as well as ever and I haven't noticed any major changes in MPG over the years as I've switched from ST Synth to Mobil 7500 to Havoline to Chevron. In otherwords, that engine is fairly stout (if wimpy), no matter what you run in it.
The little maintenance things that did seem to matter, besides regular oil changes, were to replace the PCV valve and fuel filter. Less important, but probably beneficial too, was an AutoRX treatment I did at about 90K. But again, the Ranger seems to be a pretty reliable truck no matter how well (or negligently) you take care of it.
Good luck with your truck.
M_C
PS: I think the recommendation to run MC Blend is a great idea. Seems to be great value there...