When sending queries to a mongos, the following rules apply:

For mode 'primary', drivers MUST NOT set the slaveOK wire protocol flag and MUST NOT use $readPreference
For mode 'secondary', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
For mode 'primaryPreferred', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
For mode 'secondaryPreferred', drivers MUST set the slaveOK wire protocol flag. If the read preference contains a non-empty tag_sets parameter, drivers MUST use $readPreference; otherwise, drivers MUST NOT use $readPreference
For mode 'nearest', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference