Its because a lot of standard unleaded has ethanol in it now whether the pump says it or not.
To put things in perspective today I looked at an old victa I got as a trade in on wednesday, I noticed the old fuel line was worse for wear so I replaced it, the fuel in the tank was new so I left it in there. Within 6 hours the ethanol fuel that was in the tank had started to melt the brand new victa fuel line I put on, I dumped that fuel quick smart and refuelled. Not only did it run better it hasnt attacked the second new peice of fuel line again.
