Several years for the 328 DO NOT require premium. For instance, 98.
http://www.fueleconomyone.com/1998/specifications/bmw_328i_convertible.asp
Make & Model BMW - 328I CONVERTIBLE Year 1998
City Gas Mileage 18 mpg/city
Highway Gas Mileage 26 mpg/hwy
Combined Gas Mileage 21 mpg/combined
Estimated Annual Fuel Cost $892
(based on 45% highway driving, 55% city driving, 15000 annual miles, $1.81/Gal)
EPA Class MINICOMPACT CARS
Engine 2.8 Liters/6 Cylinders Fuel Type REGULAR Unleaded Gas
Transmission Auto(L4) Drive Type Rear-Wheel Drive
http://www.fueleconomy.gov/feg/noframes/14052.shtml
BMW 328i/328is
6 cyl, 2.8 L, Man(5), Regular 20 28 $1344 8.3 NA
BMW 328i/328is
6 cyl, 2.8 L, Auto(4), Regular 18 26 $1471 8.9 NA
http://www.fueleconomy.gov/feg/findacar.htm