If I add much monotype I get the oatmeal dross. Takes more heat and a heap of fluxing to get it mixed in. Once mixed in it seems to remain stable and in solution. More antimony seems to cause the oatmeal like dross. It can be mixed in but it takes some work and patience.
Never had zinc. I rarely use wheel weights so that isn't a possible source.
That's my take on it, silvery oatmeal on the melt is more than likely antimony. With the antimony a bit high the only way I have ever been able to get the oatmeal is when heating the alloy slowly, at a temp that melts the lead/tin antimony with a much higher melting point can form on the surface. Sawdust and pot temp around 700 to 725 and it will reduce right back into the alloy.