About 6 months ago I was on a road trip, I had 16 hours on the road ahead of me, grabbed the wrong nozzle at first fill up and inadvertently added 7 gallons of 87 octane. Now thats about a 33% mixture. Thought about trying to drain some from the tank, but decided to give it a go. Thought if I could get down the road about 100 miles I could dilute it more and eventually eliminate it. I decided not to shut down the engine, not sure what effect it might have on the glow plugs.
It ran fine for about 20 miles, then starting noticing the effects: Not as peppy acceleration, at cruise, I could feel the early detonation, engine temp ran a little bit higher than normal. I was in central Virginia, and it was most noticeable cruising on the flat sections with little acceleration. Climbing the hills and coasting down the other side it was tolerable. I ran it for about 80 miles with these symptoms, then stopped to top off. Added 5.4 gallons diluting the mix to 25% (I was getting 14 mpg). Filling up must have helped mix it, because after leaving the station, I didn't feel the symptoms at all.
I cruised for another hour without feeling much, then about 160 miles, the symptoms returned. I suspect that gas doesn't mix completely, separates over time and the longer I cruised the more it separated out. I stopped after 200 miles to top off. This time when I stopped at the pump, it idled rather rough until I got fresh cool #2 into her. Added 14 gals diluting the mix to 7.5% (mpg about the same) and she idled smoothly. After this fillup, I never felt any more effects from the 87 octane mistake. Fuel mileage returned to normal.
I'm not recommending anyone run gas, just wanted to relate my experience. If I made this mistake again, I think I would drain enough to dilute to about 10% before continuing. I've put on another 9000 miles since this happened and no issues.
