I agree, you should definately change all your fluids lifetime or not, I have a 2002 passat with 75k literally runs like new. Transmission fluid change it and the filter, the small investment now goes far into the future with little or no breakdowns, all fluids are the blood the lifeline of the product they are in. My z4 has no power steering fluid unless you have the 'M' so no worries there, radiator flush, brake fluid change, differential,engine oil, unless you lease your car you'd have to be crackers to wait for 15000k to change your oil, I use Amsoil 5w 30 and am very very very happy with it. I do the maintenace myself even if it is still under warranty, fluids that is. If you lease your car and are giving it back who cares don't waste a dime extra on the car your monthly payments are enought to not spend more, but having bought my car and keep my cars 10 years plus I change all the fluids. No secret to keeping a car for a very long time, you can keep your z4 for 20-30 years if you want too, but you must keep all the fluids fresh so they can do their lubricating job.
Dustin