I've been using this solder, without investigating whether I should:
SRA No-Clean Flux Core Solder, 63/37 .020-Inch, 2 Ounce Spool https://www.amazon.com/dp/B00WDETC5A/ref=cm_sw_r_apan_glt_i_1TTK6BB98XR6EVS4BPN2?_encoding=UTF8&psc=1
However, no issues on any pedal and amp builds I've used it...