Well I think I would wait Stefan until the oily film had gone off the top of the water for tender plants but then I am one of these people that would always tender plants with tap water anyway because you never know what's in do you in rain water, anything can congregate in a pot, it can be infected with all sorts of things and I would just use this water on the garden in the first twelve months or so or use it on shrubs and things like that if it was required and then go on to er things like perennials but  then you could use it on almost anything but with the proviso that you may have contamination in that water if it's from Water Board.