Should this "new" wagon use premium gas like my 1997 E320 does?????
YES

The previous owner stated thta they used mid-grade gasoline and had never encountered problems!
Maybe they never "nailed it" and realized what it could do for them!!!
I spend the extra money on Premium. It might give slightly better mpg and acceleration.

ps, I never knew that a station wagon could move out like this one does, I nailed it on the highway and almost had an orgasm!
Maybe the PO put Premium fuel in it once they decided to sell it.