Stallman is rightfully advocating lesser use of oil, but putting a tax wouldn't solve it.
A major portion of petroleum is used as fuel that is a reason to be alarmed, we are literally burning oil for fuel value, which is a low value use & that too in engines and boilers (coal) that waste 20-60% of it for nothing, just waste heat owing to entropy. We could use petroleum and coal better for high valued petrochemicals, not just for basic disposable polymers uses. Newer materials should be synthesized with better use cases than what plastics have come to. Like kevlar, yes it is synthetic but on similar lines. I believe petroleum use as fuel should be stopped completely so now we have like a 100 years of oil left? Remaining reserves be entrusted to research use only.
https://stallman.org/archives/2015-nov-feb.html#22_January_2016_(The_low_price_of_oil)