I am the System Administrator (Standard Profile) and receiving Insufficient Privileges error when trying to access Opportunity fields.
"Insufficient Privileges You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary. "
I am doing same operations on "Lead" and it works fine as I required, but don't know what does go wrong with the "Opportunity".
I have seen "View All = True" which is by default for the System Administrator profile.