As I wrote before the CRM 2011 SDK is backward compatible, the phrase you found about RU15 is of course correct, but the meaning is that if you develop against a RU15 instance you need to use (at least) the 5.0.18 SDK if you want to use all the features
(eventually) introduced.
I make an example:
With RU12 was introduced a new message called "ExecuteMultipleRequest", this message is availiable only on CRM 2011 UR12+ instances, and to take advantage of this message you need to use at least the SDK shipped with UR12 (I don't recall now the exact revision
number) but I can use the latest SDK and use the message because the SDK is backward compatible. But it is necessary to pay attention to don't execute this message on instances not updated yet to UR12.
So you can use the 5.0.18 assemblies with your RU13 CRM instance.
My blog: www.crmanswers.net -
Rockstar 365 Profile