Actually Export/Import will work from higher version to lower version as long as the following two conditions are met --

1. You are not using any new features in the higher versions that were not supported in the lower version.

2. When exporting from higher version, use the EXP program that came with that version and when Importing in the lower use the IMP (or IMP73) program that came with the lower version

I have done it several times exporting from 8.1.5 and then Importing into 8.0 and 7.3.4

- Rajeev