Can interior paint be used outdoors?

Yes, but it won't work as well as exterior paint. The difference may not be visible in the short run though. But interior paint formulations do not include such additive as moldicide/fungicide that help paint resist wet exterior conditions, or stronger elastomerics that help paint better expand and contract with the outside temperature fluctuations, or sun protectors and special pigments that resist fading. Outside paint also create stronger/more durable surface film for better surface protection.