Having worked on several hundred heads, I have NEVER seen an austenitic seat in the exhaust position, inlet yes, most of them,only the later ones having bronze. Only ever seen bronze for the exhaust seat.According to K.T.B. exhaust seats are made from austinetic iron, the same as used by Jaguars. I have literally hundreds of clients using unleaded feuels super+regular. If there were problems I would have ques of head jobs. As none of them have materialised I can only assume that there is no need to worry on this score. The majority of vin mettalurgy was worked-out pre ww2[pre leaded ] Don't worry, be happy! Roy.