I hate to be the one that doesn't follow the trend - but there is absolutely no point in putting pentrating oil on a bolt head - it goes on threads, like studs.
On a calliper bolt, there is no access to the threads, so putting oil on won't help one iota.
http://www.golfgtiforum.co.uk/index.php?topic=49436.msg871824#msg871824
The best method has already been mentioned, get good vice grips and clamp down TIGHT, then undo it. If its the allen head bolts (as above), then you can get it off by whacking it a few times with a hammer and loosening it off with vice grips. When its off the car, grind flats onto it to bring it to 11mm and refit (or buy a new one).
If you are refering to the 13mm hex bolts - then a good quality 6 sided spanner/socket with an 18" breaker bar should make light work of it.