I have never had tree sap on my Mustang, but I have had it on my Focus several times. What has worked for me is wash the area thoroughly. Then I used "3M Finish Restorer". I used this with medium pressure and the sap polished away. When using any type of paint polishes, the polishes dries fairly quick and will need a second or third application. After each application dries, I used a clean dry cloth to wype it away and apply another until it tried. It didn't happen immediately because sap is some stubborn shit. After a minute or two it was gone. If the "3M Finish Restorer" doesn't work (it should), then try the "3M Scratch Remover". This is more aggressive and use with caution and begin with light pressure. The Finish Restorer will be needed after using the Scratch Remover.
I'm sure there are other products that work well, but these have worked well for me without damaging my paint on any car I have used them on.