I have a couple of mules that detest being paste-wormed; yet another mule loves it so much she will devour the entire tube, including the syringe, if allowed. Keep in mind, I’ve been doing this for over 50 years; this is what works.
Mules that are sour on being paste-wormed is partly due to having a syringe shoved into their mouth, the icky taste, and there was no reward offered. The handler may be nervous and that makes it more difficult.
Simply play the finger game with your mule. Apply molasses or apple sauce to your finger and slide it into the corner of your mule’s mouth. Do not rush this, give him time to think about it. Since mules are individuals, one mule may prefer apple sauce or jelly, and another mule may prefer molasses or just corn oil on your finger.
Next, reward your mule for being cooperative in the finger game. Let him nibble oats out of your hand. Mules will come off the pasture for oats.
Now that your mule is quiet and ready, keep a small cup of oats ready to use as a reward. Simply, slide your “juicy” finger into the mule’s cheek, shoot the paste and slide the syringe out WHILE messaging his throat latch to encourage swallowing the paste. If you allow the mule to open his mouth, he will opt to spit out the paste wormer. After swallowing, offer the oats, or apple sauce, as a reward and keep messaging your mule and telling him how good he is.
When grooming your mule, keep things positive by offering a handful of oats for him to nibble from your hand.