63/37 is the preferred ratio because it is "eutectic."
That $5 word just means that, once you remove the heat and the molten solder begins to cool, it goes from liquid to solid with a minimal time in a "plastic" state. It is during that plastic state that, if the joint is moved at all, that it can cause gaps and crystallization of the solder joint. (aka, a cold solder joint).
Only slightly more expensive but harder to find in stores than the 60/40. I usually buy Kester brand, no-clean rosin, 63/37.
But - there is nothing wrong with 60/40. If you have it, and it is rosin core, don't worry about it.