Duh! Figured out my own answer. Here it is for those who are interested ...
<variable name="Preparing For Your Oil Change" type="serializable">
<map>
<entry>
<key>
<string value="documentation"/>
</key>
<value>
<string value="http://oilchg.xxxx.com/oilchg-intro.html"/>
</value>
</entry>
<entry>
<key>
<string value="dateDue"/>
</key>
<value>
<string value="-8 days"/>
</value>
</entry>
</map>
</variable>