I thought I read somewhere that 63/37 solder is slightly better than 60/40, something to do with the melting point and it being more uniform in the 63/37. But I have never really looked into that to see if it has any merit, I still always use 60/40 with good results.
The only time I use additional flux is a tiny dab when tinning heavier gauge (8-10AWG) wire. Seems like it helps the solder to wick into all the strands easier and more evenly. Not sure if it's just me, but without the flux sometimes the wire seems to be a bit stubborn in absorbing the solder, the flux mainly helps to get it started.
And that is exactly what flux is for. Cleaning and allowing a better flow of the solder onto the part. 63/37 goes through the eutectic stage faster, (it doesn't stay plastic for as long) so when your hands are burning and you let off the iron it will harden faster. It does very well with wire soldering.
Jeff
The Warnings & Cautions discussed in this manual cant cover all possible conditions/situations. It must be understood that common sense and caution are factors which cant be built into this product.