There was an article in one of the HBSA journal's that covered preservation by the Leeds Armoury, an interesting article as they left steel plates out that were covered in a variety of oils, Brunox won hands down and it what they use now, previous to this it was WD40.......
As Bruce says, Sewing machine oil or as someone I know swears by 3 in one... at least storing it in the shed doesn't have Gun written on it, and if any one did get in your shed at least it wouldn't give him the thought " Gun oil, wonder were his gun is"