The Versys should take 91 Gasohol.. Look for a sticker with 91E or 95E on the tank..
If 91 gasohol is not available then put 95 Gasohol.
No point paying more tax for Benzine. As Colin said.. The bike is designed for Gasohol.
(I only put benzine in if there is no other choice out in the sticks)
If you really want to mess with your mind
http://en.wikipedia.org/wiki/Common_ethanol_fuel_mixtures
The attendants will ask you Tamada (normal) or Gasahol if you pull up to a pump with both. If anything they will try and put benzine in, you will need to say Gasohol.
Just ignore E85... Doubt they would put that in unless you insisted
91 Gasohol (green)
95 Gasohol (orange)
If not sure.. Look at the price. About 1.5 baht difference between the two.
Cheers
Brian