http://www.ecstuning.com/BMW-Z4-M_Co...2L/Search/Tws/
ECS has it for $12/liter. Their oil change kit which contains 6 liters of oil and a filter for $80 is a great price, and nice to get everything together in a convenient package.
http://www.ecstuning.com/BMW-Z4-M_Co...n_1/ES2580178/