The paint is probably a thixotropyic formulation which, basically, means it has a gel-like formula so it can be applied thicker than normal paints without running or dripping. Used mainly for non-drip paints but also has benefits for some primers where you need a thicker than normal coating. Stirring or shaking the paint breaks this gel down into a liquid and, therefore, removes this property.
The problem is that when these paints are left to rest for long periods they do, like most other paints, begin to separate with the oil floating to the top. Stirring the paint and using in its thinner form is better than trying to use it as is. You may need to paint an extra coat to get a decent coverage though.
However, try to stir it until the oily layer has gone and then leave a it for a day or two to settle.