Oil replacing butter in cakes

Most blogs say that they can be used interchangeably, but y experience is, not so.
Oil makes the cake more moist, maybe oily and sticky. One cup of butter has less fat and more moisture than one cup of oil. So the replacement formula I found here ( http://thepassionateolive.com/wp/baking-with-olive-oil-instead-of-butter/ ) is:
1 unit butter= 3/4 units oil
Plus the water content will need to be tweaked.