If there is a green coloured plastic cap over the filler plug, the transmission uses Esso LT71 141 fluid (which BMW refer to as the "lifetime fluid"). It is distributed by Exxon and the best way to buy it is from the wholesaler in a 20 litre (4 gallon) drum. Dexron is red in colour whereas the Esso product is very pale green/straw coloured (i.e. pale green when new but turns straw coloured with xtended use). My 328i coupe was built in December 1997 and uses the Esso product.