What is most efficient way to store one or two chars?
It doesn't matter much: text (= varchar) would be a little smaller, and an enum type would be a little faster (to compare).
Make the decision based on usability: if you might ever need to delete one of the values, don't use an enum. The best is probably to stick with what you have now.
As for indexing, that might just about make sense for 8 different values, but you would have to check if any of your queries might actually benefit. Without knowing the queries, it is impossible to say for sure.