That simply is not needed especially on the water boxer. Fuel system cleaners and carbon removers can also dilute the oil since some of the stuff goes past the rings and does not burn off. Current fuels do a fairly good job of keeping things clean. On the Waterboxers excessive valve deposits are usually a sign of something wrong such as worn valve guides or more common excessive piston blow-by or oil carry over from crankcase over filling. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Dan N
Sent: Tuesday, February 3, 2015 11:02 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Removing stuck O2 sensor

yes carbon build up is bad...

I use Lucas injectors/rings cleaner every other fill up the gas