In between the speedometer and tach, there are two buttons. Press the one on the right and the oil level is displayed. You can download the manual from the internet and the instructions can be found on page 88. Hope this helps.
5w-30
The oil on a BMW Z4 is changed by putting a oil drain pan under the engine, placing the vehicle on jack stands, and removing the oil drain plug. The oil is then allowed to drain, the plug re-installed, and the engine filled with oil.
It depends on what model year you are talking about. Most recent BMW's (post 2006) have an electronic oil check that you access by pressing a button on the dash. Therefore, there is no actual "dipstick" in the engine bay.
5w-30
It's in your Owner's Manual.
Check KBB.com or Edmunds.com
The 2012 BMW Z4 has 16 valves.
The 2005 BMW Z4 has 24 valves.
The 2009 BMW Z4 has 24 valves.
The 2008 BMW Z4 has 24 valves.
The 2011 BMW Z4 has 24 valves.