FWIW I tried 87 octane recently in the K75 (generally use 93 octane). I noticed more knocking under load at very low RPMs (e.g. slipping the clutch and pulling away from a stop sign in 2nd gear). I thought I might be imagining things, but when I switched back to 93, the knocking was all but gone.
I don't like that knocking sound, so I'm going to go back to 93. Maybe I'll try 91 some day, but the price difference is so small over a few gallons that it doesn't make much of a difference.
