A rag 'moistened' in petrol might work as well, but you will have to re polish the car, as all of the existing polish will have been stripped off.
Take it to a bunch of Polish (double meaning :- bloked from Poland. Who polish) blokes under some arches, and give them £40 for a full valet, and make sure they know the tree sap is to be dealt with properly. Alternatively, find a 'detailing' geek and flutter your eyelashes at him for a bit.