At the start of June, oil targets were made a priority of night bombing with attacks on other war industry to be made on dark nights (when the oil targets could not be located) but with the proviso that "indiscriminate action" should be avoided. On 20 June oil targets were made third priority below the German aircraft industry and lines of communication between Germany and the armies at the front. Following a brief period when German shipping was given priority, oil targets were made secondary priority in mid July under a policy of concentrated attack with five oil refineries listed for attention.[10] Sir Charles Portal was sceptical of the likelihood of success, saying that only a few targets could be located by average crews under moonlit conditions.