Not a silly question. I've seen 'sugar free' ones before so I assume they contain less syns.
Also, do they have the nutritional information on the side of the bottle? Calories, protein, carbs, fat etc? If so, you can use the calculator to figure out the syns or post the info on here (with the serving size) and someone can calculate it for you.
But, to be honest, if you're ill - you're ill. I say sod the syns and take the medicine if you need it.
Just got my book out in the syns section it says 2 level tsp of cough syrup is 1.5 syns on either day doesnt have a brand so guess it just applies to the majority , but to be honest i agree with Ste if your ill your ill sod it just take it your less likely to turn to food for comfort if you dont feel like cack