Depends on what type your paste is. Type 3 and 4 are pretty common but may be hard to push through a 24 ga needle. I started with T3 and found it to be difficult to use with smaller gauge needles, even with teflon coated needles. Of course, those are needed for LGA and QFN parts because you want fairly small solder paste dots. I tend to use stencils now so T3 is ok. Next order, I'll get something smaller even though T5 is about double the cost.
To clean, I use IPA (not india pale ale) to clean them with a spare syringe.