Colin be careful with injector cleaner. It made my perfectly running '94 E320 wagon stumble and shudder for a little about 3 days. I thought I'd put some in as preventative maintenance - but no more. The theory I've heard is that is brings up muck from the bottom of the tank into the gasoline and you can guess what happens next.
I'm starting to figure out that most additives and things sold at Auto Zone are to keep a lousy American car on the road. I could be wrong about all this (and my wife says I'm usually wrong) so follow up with a search maybe.
1994 E320 Wagon (Died @ 308,669 miles)
1995 E300 Diesel (194,000)
1999 E300 Turbodiesel (244,000 miles, dies at 255,000)
2006 Toyota Tundra SR5 AC 4X4 (115,000 miles)
2011 Audi A4 Avant (129,000 miles) hate this car!
BMW 330 xi 6 speed manual (100,034 miles)