Re: an unpopulated 40-pin GPIO interface
60/40 is much easier to use, but it does not mix at all well with any of the Lead-free solders, so if you are making changes to a manufactured part which uses Lead-free (i.e. pretty much anything made in the last 15 or 20 years) you must also use Lead-free. Obviously the same applies in reverse to older kit.
It's not good practice to use the same soldering iron tip for both, either. I have found that tips are damaged very quickly if you swap between the types. From a hobyist's perspective it's best to stick to Lead-free these days, but if you are working with a variety of kit you will need to keep separate tips, or even two irons to avoid having to swap tips.
Make sure you get a Lead-free solder with a flux core. It seems to be far more common to find Lead-free without a core than it used to be with 60/40. There are also many different formulations, and I have found those with a high Silver content best, but this might just be down to my personal style.
Lead-free solder isn't quite as "eutetic" as 60/40. WIth 60/40, once you take the heat away there is a brief cooling period where it stays liquid, then it all turns solid pretty much at once. Lead-free seems to go through a short "solidyfying" phase, where it goes goey. In other words, make sure to keep the joint still for longer than you'd expect with 60/40.
But the biggest tip of all - and this solves nearly all the problems that "hobbyists" have with Lead-free solder - is to use a powerful iron (at least 40W, but this depends on what you are soldering) which can be set to reach a tip temperature of perhaps 350C - considerably higher than you would use with ordinary Lead solder. If you are soldering correctly you will apply heat for such a short time that it won't damage the component you are soldering, but it does take a bit of practice.
Put it this way, a few years ago I ran a successful "build a kit" workshop with children (mostly 8 - 12 years old) who had never soldered before. After a couple of trial runs, all of them managed to solder together working kits and I only had to "remake" one or two connections for a few of them.