Take the boots to a shoe shop, leather shop, etc. and have them rubbed in with saddle soap and then a sealing oil. This will help eliminate spots and make them uniform.
You can try putting baby powder(talcum powder) on the stain, covering well. Let sit over night or longer. Brush away the powder. Repeat if necessary. If all the powder does not come off, tumble boot in dryer on air/no heat cycle with an old towel to remove powder. I used ths method to remove salad dressing stains from a suede skirt, and butter stains from a jacket. Hope it works for you.
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