Adding it (at a very thin ratio) will be ok in a car - about 1/4 of a gallon per fillup, I've done this many times. As mentioned earlier the ratio would be so small it wouldn't be noticable. I mean I would not do it every fillup forever but once in awhile to get rid of a gallon or so is fine. If your nervous about doing this in a car the same holds true in a mower.
Oh ya, just my .02 please don't call me if you ruin your engine.
But seriously, I've done it before.
