I haven’t had any trouble with 'gunk' using my 100% wool mat on high heat, the standard setting i have my iron on when quilting. I suspect in a horse saddle pad there maybe some variance in fibres in there, like a little polyester that may be melting onto your iron. Not critical for a horse saddle pad, but would be when ironing on it. I don’t use spray starch with the wool mat, but do use spray starch on my ironing board if I feel I need to, then once the starched fabric is pressed dry, I will press it when in the piecing process, on the wool mat and don’t worry about it.