So I was looking at all synthetic ATFs that officially claim they can be used in place of Honda ATF Z1. While comparing them I came across two ATFs that have distinctly lower VI than all others, yet their other specs are similar. Amsoil and M1 were the only ATFs that have a VI of ~170. All others (RP, RL, Castrol IMV, Pennzoil) have a VI ~200. Now M1 and Castrol IMV are the only ones that claim "exceeds JASO 1A requirements". and Amsoil and RP have the highest flashpoint, and RP also has the highest VI, but none of them officially claim "JASO 1A". So are VI and the JASO requirements both equally important for selecting an ATF as a replacement for Z1 ? Or does one outweigh the other ? It seems like Castrol IMV is the only choice if you want to consider both a higher VI and the Jaso requirement. Thoughts anyone ?