How to set up roles Data Sharing Options so that a user can view but not update each other’s data (i.e. Read Only)?

How to set up roles Data Sharing Options so that a user can view but not update each other’s data (i.e. Read Only)?

Hi,

I have an admin user and a couple of sales staff, setup through roles. I would like the admin to be able to change anything in the CRM (change the sales staff leads, potentials etc) but I would like the sales staff to be able to only edit their own leads and potentials (only the ones they created), but can still view all the leads and potentials in the CRM. 

I saw in the help files that there was an option for Read Only in the Data Sharing Option but I could not see it in the dropdown of options (only had Private and Read/Write privilleges). Thanks :)