Update Macbook to newest supported OSX version? My Late 2008 Aluminium Macbook has 10.9.5 on it. The update tool offered me Sierra but then predictably told me I can't have it as this Mac is too old. I don't know what the last supported OSX version is for this Mac... is there a way I can get it?

In an ideal world I'd like to do a clean install as this MacBook is full of all kinds of crud I don't want anymore.
The latest OS supported on a Late 2008 Aluminum MacBook is OS X 10.11.6 and can be downloaded from this link which will open in the App Store.

Note that while is starts off saying "This version of OS X El Capitan is for users running OS X Snow Leopard...", it also says...* "This version of El Capitan can also be used on Mac computers that are not compatible with Sierra.".



You can use the instructions in Create a bootable installer for macOS to create a USB Installer to do a clean install of OS X El Capitan.


  Use the 'createinstallmedia' command in Terminal
  
  
  Download the macOS installer from the Mac App Store. Quit the installer if it opens automatically after downloading. The installer
  will be in your Applications folder.
  Mount your USB flash drive or other volume. You could also use a secondary internal partition that has at least 12GB of available disk
  space for the installation files.
  Open the Terminal app, which is in the Utilities folder of your Applications folder.
  Use the createinstallmedia command in Terminal to create the bootable installer. For detailed usage instructions, make sure that
  the appropriate macOS installer is in your Applications folder, then
  enter one of the following paths in Terminal:
  
  
  Path for El Capitan:

/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia

  
  Example
  
  This is the basic syntax of the command. Replace volumepath with the
  path to your USB flash drive or other volume, and replace
  installerpath with the path to the Install OS X app. 

createinstallmedia --volume volumepath --applicationpath installerpath

  
  The following examples assume that the OS X installer is in your
  Applications folder and the name of your USB flash drive or other
  volume is MyVolume:
  
  Example for El Capitan:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app



After creating the USB Installer, you can boot the Mac with it and do a clean install.  Make sure you backup anything you do not want to loose before erasing the HDD and installing OS X El Capitian.