Can you split the 5 gallon bucket and then hold the 2 halves together with bungee cords or ratchet straps and then release and split the halves?
PAM is soybean oil. Worth a try in concerned about the environment.
Otherwise anything that makes the inside plastic very slippery might work, consider sealing the pores with rainX (makes everything slippery!), quick coat of non-synthetic car wax, or silicone spray.