Prevent user from changing picklist field if not Opportunity owner

Hi,    I am searching for a solution for the following:  I have a set of custom fields on the opportunity object, some of them are picklist fields and  I would like to make sure that only the opportunity owner can edit these fields.    At first I thought of using a simple validation rule with ISCHANGED and $User.Id <> OwnerId, but forgot that picklist fields are not supported in validation rules.  So that doesn't work.    Any suggestions would be highly appreciated. Thanks