A tip to keep the oil from separating: I add about 5ml of water to a jar of peanut butter and stir in well. This creates an emulsion that thickens the peanut butter and prevents the oil from separating out. It improves the spreadability as well.
Save yourself a huge mess and don't try using an electric handheld mixer to do the initial oil stir...I got peanut butter EVERYWHERE. It's a nightmare to clean.