RBF 600!!
Well it's actually worse if it's not used. 2 years would be ideal though IMO
It's gonna be down to personal preference - but I know 2 teams who entered WRC last year running it
The only difference is the boiling point
5.1 is higher than 4
Super blue is a fair bit higher than 5.1
and RBF600 is higher than super blue
But on a little clio road/track car even with 4 pots - you'll never ever ever reach the boiling point of super blue
It's not in my eyes worth the extra money and the yearly changes