I'm afraid I might actually be the most likely person on this subreddit to be able to answer this question--I hope I'm wrong, because it's a mystery to me.
Título 21 of the Fuero of Sepulveda reads:
"Mando que ningún omne non pendre a ningún omne que viniere con miera a Sepúlvega, siquier sea christiano, o iudío, o moro, si non fuere debdor o fiador; et sil’ pendrare, peche al conҫeio C mrs. y al querelloso los pennos doblados.”
So basically, anyone who comes to Sepulveda with miera is not to be held. As far as I can tell, miera appears to be cade oil. It seems like an oddly specific and non-essential product to require specific mention in the fuero (it is neither preceded nor followed by similar injunctions for other commodities), so I'm wondering why this is in the fuero.
I checked it against the Fuero of Cuenca (upon which the Fuero of Sepulveda is based), and it just says mercaderias (merchandise/commodities), more generally.
So, hopefully someone with more extensive knowledge on medieval commodities/trade can give me some guidance here. What's so significant about cade oil? Or am I misinterpreting this titulo?
Maybe its not cade oil that they feared, but turpentine. The wiki says that it can "irritate the skin and eyes, damage the lungs and respiratory system, as well as the central nervous system when inhaled". Also, it is highly combustible.
The RAE defines miera as both cade oil and turpentine.
One minor correction to your interpretation of the titulo: it seems that the law allows for the possession of miera in the fuero if the man either owes or is the creditor of such good. "Do not let anybody with miera pass, not even if he is christian, jewish or moorish, unless he is a debtor or a creditor (si non fuere debdor o fiador)".
Hope it was of help!