Hi, Al,
I'm currently using
TermoPasty Topnik TK83, which is classed as a medium-active rosin flux, a bit more aggressive than pure rosin/alcohol. In liquid form, it comes in a wee bottle with a handy brush in the lid. You'll find it on Ebay.
It says that it's no-clean, but I like to clean up anyway. The very best cleaner that I've ever come across was carbon tetrachloride (CTC), but they decided it was pretty nasty stuff carcinogen-wise, so they issued trichloroethane; then they found that was pretty nasty as well. I use IPA these days (the solvent rather than the beer!) but it takes a bit longer, particularly on old, hardened rosin.
And David, you have me questioning my memory as well; thinking back 50-some years, it may well be that we did just use multicore solder in training and that the flux thing is something I've picked up since. Works well for me, though!
Cheers,
Frank