Some forts, I have no problem cooking syrup; so long as there are barrels of it available, my cooks will happily take a single solid item and 3 syrup barrels to make lavish meals.
Other forts, cooks will refuse to ever use syrup if there is anything else available anywhere on the map.
The only science I'm aware of on the topic is the observation that cooks will always prefer solid ingredients in pots/barrels to liquids.
However, this is not the only factor involved; in fact, I just did an experiment where I made a fort whose sole existence in life was to produce sweet pods and quarry bushes (I used growcrop and fastdwarf if that matters) and try to get cooks to make dwarven syrup roasts automatically.
I placed a ring of dwarven syrup around my kitchen and then placed a stockpile for leaves (no barrels!) elsewhere. Result: cooks always took leaves if they were available.
I tried many different placements for the leaf stockpile relative to the kitchen (and even tried without stockpiles!) -- e.g. to the North, East, South, even a pair of stockpiles to the North and Southeast. Sometimes I linked the piles to the kitchen, other times I disabled the links. I used meeting areas to get my cooks start the cooking task from different areas of the map. But every time, they would simply take leaves until they were all gone.
What do I need to do to do this?