Looking for formula help for a lead.  I need a field called "RSD Lead Owner" that auto populates with the sales rep assigned the lead when they are of the role "RSD Sales".  (I assume using the role is required to  eliminate marketing and admin from the equation as they are sometimes the first lead "owner" before they officially assign the lead.)  

Once this field populates with the rep in the role of RSD Sales I need the RSD Lead Owner field to remain this owner no matter how many times the lead changes hands and is assigned to someone outside of the RSD Sales role. 

Essentially: RSD Lead Owner would equal the first person assigned as the lead owner in the role of RSD Sales.