WAS 8.5 is not supported yet, but is on the road map for another release. Splunk app for JMX is another app that also works like WAS. https://apps.splunk.com/app/668/. The first release does not support all the fields, but the next release will support multiple fields with the same name in the same event.
Version 3.0.0 of the Splunk Add-on for IBM WebSphere Application Server is compatible with the following software, CIM versions, and platforms.
Splunk platform versions 6.0 and above
CIM 4.2 and above
Platforms Platform independent
Vendor Products IBM WebSphere Application Server versions 7.0.0 - 8.5.5