there are always two options you can choose:
1) upgrade your Pro*C environment to Oracle10g client / Pro*C,
2) run your Pro*C8 programs – use an SQL*NET connection to your Oracle10g database or set the variable (on UNIX) TWO_TASK=<database alias>.
The second option – be sure to check if Oracle supports connections from Oracle8i client tools like Pro*C. You may run into problems with such connections and experience various errors related to SQL*NET Oracle8i connections to Oracle10g database.
I hope this helps,