Maybe I don’t understand “rntime”, but Zulu is still the recommended Java to install according to the openHAB docs, as @pleedell pointed out.
Zulu is currently the recommended Java platform for openHAB.
[…]
OpenJDK may also be used, but it has some known limitations with openHAB and is not recommended.
(Irrelevant, of course, when talking about openhabian)