Turned over 87,000 miles last week while on a road trip, and noticed that mileage was a little off (16.3 highway, 14.2 mixed city/highway).
Since I'm closing in on the magic 100K service, decided to spend the Saturday playing in the shop. Air filter, rotate tires, BG 44K and injection cleaner, flush transmission and replaced with Dexron 6, pulled front and rear differential covers and swamped out the diffs. What were the engineers thinking when they cam up with the diff fluid spec: 75W90 Synthetic in rear, 80W90 standard in front? Replaced front and rear with 75W90 synthetic; figure erring toward synthetic was not too bad a decision.
Replaced all 8 plugs with new AC Delcos. All 8 came right out with minimal effort, a couple were pretty slow so I hosed those down with a penetrating oil.
Overall very pleased with condition of the plugs. I'm old enough that I can remember a time when I would have been pleased to have plugs look this good at 20,000 miles.
Results? Ran 140 miles out and back at interstate speed today for another day of work, and averages 20.9 mpg door to door with about 18 miles of that being in town.
Yup, made me happy.