Lead-free solder is a pain so go for lead solder and a good flux.
http://www.amazon.co.uk/gp/product/B...=1&*entries*=0
http://www.ebay.co.uk/itm/17079-Elec...-/281419054093
Do not use any flux paste with acid in, however small the amount. Clean excess flux off the joint with motor cleaner or similar.
Always use the flux on new joints to first tin them and then make the joint. Once the joint is made use the flux every so often to clean the joint. Repeated soldering of motor wires will create oxides that can give a dry joint. Re-tin the wire and post from time to time to keep the joint clean.
The resin-cored solder that is available isn't good enough. The flux evaporates almost immediately it is melted, so all it really does is clean the tip, not the joint! HTH
