My '99 100 is quite happy with regular 87 octane. It calls for premium and I have run premium but can not tell the difference in over 3-years/51,000 miles of experimenting...except in how much it costs to fill up ;-).
I live at 5,000' and unless you're force air induced your wasting your money on higher octane...the air density just isn't enough to make the math work. Also at sea level it runs fine...no issues what-so-ever 'downstairs'.
Mine is heavily laden/accessorized along with 4.88's, 35" MT/R's, etc. and it pulls 13.5 around town. It will get 15mpg on the highway...and a little better if I keep it close to 2,000rpm (the efficiency sweet spot for this engine) which is typically 60-62mph I can get 16mpg (if I blast along at 80mph then it drops to 12-13mpg). Not bad for a big heavy rig with 4.88's, full MT tread and full time 4WD! Of course having said that...20mpg would be better ;-).