I would start with a green pan scrubber - it it didn't work I would mix up a small solution of caustic soda, carefully paint it on to the stain and wait 20 mins. & then wash it off.
Caustic soda (sodium hydroxide or lye) is very corrosive and will strip paint in seconds and do very nasty things to skin and eyes so if you go this route take great care. Keep it away from aluminium and plastic - it's OK in a glass jar but if you use a brush with nylon bristles they too will melt.
From wikipedia...
Solid sodium hydroxide or solutions of sodium hydroxide will cause chemical burns, permanent injury or scarring, and blindness if it contacts unprotected human or animal tissue. Protective equipment such as rubber gloves, safety clothing and eye protection should always be used when handling the material or its solutions.
Dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables.
Peccavi, July 2009
