I'm trying to decide between Castrol or Valvoline import multi to use in my 2006 Highlander with Aisin 3309/T-IV transmission. I used the Castrol in my Honda Element for 100K miles with zero problems. However, I noticed that the viscosity index for the Castrol is 200, where the Valvoline, a synthetic fluid, is closer to 150. This seems like a huge difference for two products that are essentially the same price. The question I have is, what is better in an ATF, higher or lower VI rating? For reference, the original Mobil 3309 fluid has a VI around 185.