Ack, too much premix in the tank!
 

This weekend I was at a track event trying to get my instructor license, and I went to the pump to top off my tank with 100 octane, so I dumped in the 8oz of premix that I thought I'd need, only to find that there was a padlock on the 100 octane pump! I ended up not driving my car for the instructor school except one run in the afternoon, and I was taking it easy during that run. Afterward, I noticed that my boost gauge was showing only 12" of mercury instead of the usual 14" at idle. I was afraid my motor was getting tired and might have to be rebuilt soon. When I got gas for the drive home I made sure to figure out an appropriate amount of oil and gas to get back to the normal ratio, and when I got home the idle vacuum was back to 14". I wonder if too much oil in the mix was over-lubricating the seals and causing them not to seal as well as they should or something like that. Just thought I'd share.

Others on the forum have had troubles with adding too much oil when pre-mixing. Usually they weren't able to keep their cars running with such "over-saturated" mix.....Others have also said that premixing lowers the octane rating of the fuel a bit...maybe that has something to do with why your car's not running the way it normally does

It definitely does lower the octane, hence my taking it easy and not going for max boost and RPM. I think it hurts the sealing and lowers compression, which would hurt the ability to keep running. Lower octane alone wouldn't really keep you from running or idling well, remember that the NA cars like 87 octane.