IIUC, it's both, but you're right lubrication is not it's primary purpose, but I think the reason leaded fuel is hard to replace is that the lubricating properties are difficult to replicate.

> wherein it served as an effective antiknock agent and reduced exhaust valve and valve seat wear.

https://en.m.wikipedia.org/wiki/Tetraethyllead

That's mostly an issue on cheap engines without hardened valve seats. Aviation engines switched to hardened valve seats well before WWII.