You should be logged on as a user with all of the environment variables pointing to your 8.1.6 database. 8.1.6 and 8.1.7 should have been installed as and run as different users.
You can even specify the physical path for the exp utility i.e.
Also make sure that your 8.1.6 database is still running.
You can always create another user for 8.1.6 and setup that user to have the proper environment variables in its .profile file. Then you can log on as that user and do the export. You may also want to start the database as that user.
I guess you know by now that you should have done an export before installing Oracle 8.1.7 and also installed 8.1.7 as a different user.
I deal with smaller databases and I don't normally setup a machine to be a multi-homed machine. I get the export uninstall the old version and install the new version of Oracle and then do the import.