You are correct. I was overthinking - but this thread was educational and I feel much better since I started this thread and getting good advice from fellow Jeepers.I think you may be overthinking the situation. To the other's points, I'd just use the 87, then refill with 91 when your tank is empty. On a related note, my daughter's 2004 Volvo XC90 requires 91, however, we've been putting 85 in it for years. It's a 2.5L 5 Cylinder Turbo motor. Runs fine on the lowest grade. Also, my previous BMW's required premium. Of course, I liked to put premium in, but again, but have put in lesser grades. Also ran fine. My 2 cents.