Old engine oil used to be the cheapest and most effective but I believe it's now banned. It also used to give an interesting glittery effect if the engine it came from had gone bang and left tiny particles of metal in the oil.
ours was done 50/50 mix of engine oil & fence preserver & even the fence munching, crib & windsucking pensioner hasnt bothered since. one quice sniff & wandered off.
looks fab but for the first few days after it was done my boy came in with black smudges up him (but he is grey with a keen interest in colour changing!)
note that creosote is banned for purchase by joe public but farms are still allowed to buy it, altho nasty chemicals in it, it is far better as preservative than any of the replacers (and prob no worse than engine oil!) and of course does no harm if its taste actually stops the horse chewing the wood in the first place!!
so might be good to find a friendly farmer to get some via.