JRE version 8 will indeed work with OneClick on Spectrum version 9.3 and 9.4. I've been running on JRE 8 (on Windows 7) for quite some time with no issues.
There are some things you need to do to allow it to work.
1) in the Spectrum OneClick Administration>OneClick Client Configuration page, make sure you have enabled the latest version with the "+" (to "Allow new versions*") and save.
2) If you need the JCE Unlimited Strength Files for JRE version 8, you can download them from Oracle using the following link:
Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files for JDK/JRE 8 Download