Solder type question

OK, i’m new to soldering and I have everything down except what type of solder to use. I’ve searched around and done some research but i’m still a little confused.

I get that 60/40 is is typical and that to use a rosin core, but i’m not getting the whole “no clean” thing. I bought some solder (Qualitek 50-6000) and everything looks good except it says it has a no clean core. But then I read somewhere to not use “no clean” solder…can anyone help?

What was the argument against no-clean solder?

I’ve used good old Kester 44 forever and rarely cleaned off the flux. Just tossed out some old projects that were probably at least 25 years old, and have been in the basement all that time. The copper on the PC boards was still shiny and no evidence of corrosion that I could see.

Anyway I wanted to get a thinner solder (0.020", probably too thin), so I went with Kester 245. Would appreciate any experiences or opinions on it.