I'm sick of our heritage being pickled in aspic. Stonehenge has been around for quite a while, and it's taken it's fair share of knocks during that time (hell, didn't Victorians chisel off keepsake to take home). Historically, some of the biggest 'vandals' have been bloody archaeologist re-erecting stones and using concrete props etc... Things like the oil will degrade over time, even if it isn't breaking down straight away. I am horrified at how people behave at these sites, but I still agree to access without paying, to all sites.
Historically archaeologists/antiquarians have done a lot of damage not just by the use of cement (although not sure an effective alternative has yet been found) but much more so by excavations where prehistoric barrows have been left partially destroyed. None of this has a bearing on protecting Stonehenge - I cannot imagine why anyone would want to pour oil on the Stones in the name of celebrating the Solstice. This sort of wilful damage should at least carry a substantial fine.