I have seen or read about so many choices for fluxing, that it's hard to track. Like most, I stop looking when I find something that works or gives me good performance. I think you can use just about anything and it might work. ..
Parafin comes to mind and read where a fella used old broken crayons. I think a key point is to try and minimze the smoke. ..
I too have tried many things but lately I pretty much stick with Frankfort Arsenal's flux as well as their mold release. A little seems to go a long way and still working on my second jar. I would add that I smelt and cast at different times and use separate equipment/supplies. ....
Be Safe !!!