During the 70s and 80s phase-out of leaded gas, hardened valve seats were installed in gas engines.

Up to this time, what did diesel engines have to lubricate and protect the valve seats instead of tetraethyl lead...did diesels ALWAYS have hardened valve seats?

Masochist to Sadist: "Hurt me."
Sadist to Masochist: "No."

I think that the Tetraethyl Lead was to reduce 'Pinking' and allow higher compression ratios, not to assist with lubrication.
regards
dva

I am not sure of the root need for the lead, but engines designed for leaded gas need to have hardened seats put in before using unleaded gas.

Keep everything as simple as possible, but no simpler

--Albert Einstein

The lead acted as a "cushion" to help keep the valve face from beating up the valve seat so much. Lead also increased the octane rating some how. Propane engines have extremely hard seats, propane has no lubricating tendencies to it.
When doing a valve job on a pre 70's head the exhaust seats must be replaced with hardened seats or failure will occur in a short time. Interestingly enough, only the exhaust seats need to be replaced due to the higher operating temperatures. The intakes will be fine.
Diesels have harder seats than gas engines. I think this is due to longeivty more than temperatures. The diesel burns much cooler than the gas engine.
Wags