Should I be adding a lead subsitute to my 914/4 2.0's gas.

No. Aside from the octane boost it provides, lead was present in gas to primarily protect the valve seats on engines with cast iron heads, where the seats were frequently just cut into the relatively soft iron. All engines with aluminum heads (and all new cast iron-headed engines) have hardened seat inserts installed that don't require lead. Indeed, lead contamination can actually cause problems by forming deposits and clogging things like injectors or carb jets.

Just so you don't feel ignored, the last post was 100% correct. Porsche and VW have used hardened seats since the 50s (and pretty hard ones even then), and you don' need no steenkin' additives ...................... The Cap'n