From the Super Mario Wiki
Royal Syrup is a powerful type of Syrup featured in Super Mario RPG: Legend of the Seven Stars. This golden-yellow syrup comes in glass jars. As Max Mushrooms are to HP, Royal Syrups are to FP. When used, this item will recover the party's entire supply of FP. Of course, Royal Syrups (like Max Mushrooms and KerokeroColas) should be saved for the most extreme of circumstances, such as difficult boss battles. This rare item is not sold in any shop on Mushroom World.
Jammin' Jelly, an item from the Paper Mario series, is very similar to Royal Syrup. Both are extremely rare, and both are the most powerful FP-healing items in their respective games. However, Jammin' Jelly only recovers fifty FP, while Royal Syrup recovers all FP.