I am about to change the automatic transmission fluid in a 2005 Honda Civic LX with over 140,000 miles. I would like to use synthetic fluid, but my only options are Mobil 1(not ahonda approved), Royal Purple Max ATF(I'd rather not spend $15 a quart if I could find a comparable alternative), Valvoline Maxlife(I do not know if a viscosity index of 150 is high enough) or Castrol synthetic(also has a low VI). For the car listed above, is a high viscosity index important? Will it noticeably benefit from synthetic? If not, I am between Castrol Transmax High Mileage, Castrol Transmax Import and Pennzoil Multi Vehicle. Which is best? Will synthetic make a noticeable improvement?