Oracle does permit backward compatible imports as long as new features are not used. You must install the Oracle client for 8.1.7 on the 8.1.6 server and use these tools to do the import.

Or you could export from 8.1.7 using the 8.1.6 client and just import like normal on the 8.1.6 server.

(btw, when I installed the 8.1.6 client software, not all of the executables were installed an I needed to manually copy them.)