I just make straight butter (ie I don't add any oil, water or salt) and so long as I've rinsed out ALL the buttermilk, it lasts on the bench for up to 5 days, and that's in Adelaide so not exactly a cool climate! I just keep it in a glass container on the benchtop and it's the most delisious, smooth and creamy butter ever. I think you only need to add the oil if you want it spreadable from the fridge, but I prefer not to refrigerate mine.