SOLDER Help

I have an acquaintance who uses a exotic solder that on a/b comparisons beats anything I have ever heard. He willingly will solder up cables and connections for me, but will not reveal what this solder is. Finished joint/connection is a dull grey, not bright and clean as you would normally expect. Could also be an exotic FLUX/cleaner. Would like to do my own soldering, as his time is very limited, and sometimes the wait is considerable.

Would appreciate any suggestions or leads. Thanks: