Short answer is no. Since inception Creo has been always forward compatible but never backward. A single exception existed briefly for a single release a long way back. I think it was R18 and R19.
As a workaround you can export the Creo 8 files in the Creo neutral file format and then you can open them in an older version. You will not have access the model history when using this technique.
Install Creo 6 and use the same license file you are using for Creo 8. This will enable the use of Creo 6 or 8. If you want 6 only then remove 8 and install 6.
You can download here.
Why do you want Creo 6? It is at end-of-life and has no further support.
If you must go down from Creo 8, use Creo 7 which has a 3 year life, until Creo 10 comes out in 2023.
Prior to Creo7, there was a GCRI that you could use to open newer files in an older version, limited to 2 or 3 versions. This plug-in was dropped at Creo 7 due to the data structure changes for multi-body parts.
GCRI = Granite Cross Release Interchange