Hi Phil,
this is not supported (and possible), for the simple reason that the synchronous API is a wrapper around the asynchronous one and as a result the async API has no clue about the sync one.
If you need both access I recommend you to use the sync bucket and then chain in an .async() one where needed, or if you have mostly async calls but you want a few blocking ones, just block on the observable right away (which is what the wrapper does aynways) like: .timeout(…).toBlocking().single();