In Pathfinder, in the magic items chapter it discusses oils under the potions section. An oil is essentially a potion that can be applied to whatever target you can interact with, following the rules for potions. The brew potion feat does not explicitly allow you to make oils. Is it safe to assume if you can craft potions that you can also craft oils?
I think the main difference is application method (swallowed vs. rubbed on one's belly). Really, I'd suggest that throwing a handful of herbs/alchemical ingredients into water would make a potion, whereas those same herbs/alchemical ingredients into oil would make an Oil.