When you look at what is n-butanol , it's an extract from ethanol and could be used as an additive in automotive fuel/gas , so if you have time you can make your self the test on piece of metal : put some 518 and drop your mix on and wait to see if it acts on the bad way ??
I had just bought some 518 (following the forum advise ) not easy to find in France at a fair price , previously was using permatex moto seal which is fuel resistant (but cannot get it any more !), so I will follow your post with attention !
PS : I was not in the chemistry business , tho work on chemical tankers at the end of my sea life !