Recommendations not based on oil analysis are just opinions. For the Vanagon especially the Waterboxer with an intact air filter system dirt in the oil usually is not an issue, unless the air filter fails. However, even with synthetic oils the oil is subjected to oxidation and nitration probably due to the high upper piston ring temperatures. Still 7.500 miles is usually a good upper limit for oil and filter change intervals. There are many variables that will shorten or lengthen this requirement. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Dan N
Sent: Sunday, February 21, 2016 9:11 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Tune Up Poll

my mechanic recommends 7K miles oil change using synthetic oil for the sake of oil filter that get dirty rather oil performance....