With respect to transmission problem, first 03 ES300 I test drove, tranny was awful. Salesman tried to BS, so we left.
Different dealership, different ES300, smooth as silk. Purchased that one (Leather, Wood kit, memory, trunk mat, HID, Sunshade, DVD Nav, 6CD changer, but not Mark Levinson). 5k miles still smooth.
With respect to gas, cars will tell you what they want. Knocking, pinging etc. Todays computer systems can adjust mixture and performance to adapt to different fuel types. My 96 I30, knocked on regular, my 03 ES300, runs smooth on regular. Not a peep. Tried premium, couldn't notice a difference.
Don't let TV advertising make you think you need to spend more money. Those cleaners etc they add, its one can in a whole big tanker.
Friend in gas delivery business (Port Everglades, Ft. Lauderdale, Fl) says he delivers for all the big guys. Same tanks, same tankers but depending on company, he has to add a few gallons of additive before he leaves the port. As he said "ever notice the tankers dont have names on them anymore, just silver with pop on signs". Use regular, $1.45 today. Wish I lived in Carolina's where it's even lower.