Perhaps the thickness of the syrup was not a significant enough difference to prove this.
Pressure is pressure. No liquid is going to react differently to pressure applied by your head and shoulders than it does to pressure applied by your arms and legs. In other words: swimming is essentially pushing against something with your head while you push against something else with your arms and legs. The pressure and force received from these two actions should stay comparably constant in any PURE substance.
So maybe you would swim at a different speed in mud, if there were rocks in it.
can you 'coast' or 'glide' in water?
can you do the same in molasses? How about caramel?
For example, scallops can swim in water, but with a high enough viscosity (or a microscopically sized scallop), their swimming style would get them nowhere. See http://en.wikipedia.org/wiki/Scallop_theorem . At high enough viscosities, the best way for a human to swim underwater would be to grow new skin on the scalp and reabsorb excess skin at the feet.
Viscous fluids are better at stealing the effort you've put in, so you're going to have a harder time swimming in them, unless you can find a good way to push yourself along without donating your velocity back to the fluid.
Since your hands move faster than your head, I think that means you would swim faster in such a liquid, up to a point, although after a certain point swimming faster gets harder to. It hurts my head to think about it though.