I say apparent as that was the way I was taught in college, and if you Google the term it is in use. Generally the base octane rating is how the fuel under test compares to straight iso-octane (and comparatively against heptane) with no additives, and then when antiknock additives are added it increases the "octane" rating, which isn't due to the base molecules but instead the additive. I've always seen that listed as the apparent octane rating in literature or descriptions. So although the end result is the same (a printed octane rating on the pump label) it distinguishes between the actual octane rating of the hydrocarbon base and the observed performance of the finished fuel in the test engine.
And yeah, there is no need for lead whatsoever in today's automobile engines and no need for additional lubrication. Even if you did have an old engine that had soft seats there's no indication MMO would provide the same benefit as lead to reduce the welding you mention. My point being that lead wasn't there just running around "lubricating" stuff in the engine. If anything it was largely a detriment and caused other issues which were eliminated with unleaded fuel.
Originally Posted By: bunnspecial
Yes, TEL was initially added to increase the octane rating(not sure why you say "apparent") but its effect on valves is real and well documented.
Exhaust valves run very hot for a couple of reasons(funneling hot exhaust gasses and no benefit of fuel/cold air flowing over them to cool them) and opening and closing against a plain cast iron seat will "micro weld" every time the valve closes. When the valve opens, this has to be broken and material is slowly lost from the valve face. This is what causes valve recession. It usually will hold for a while, but once recession sets in badly, the valve seat goes pretty quickly-I've seen seats move a 2-3 thousanths every few hundred miles.
TEL breaks down and causes lead to "plate out" on the valve face and seat. Since it's "squishy" it helps the valve seal, cushions its closing, and is sacrificially pulled off in each micro-weld event.
BTW, I've heard old mechanics talk about how Amoco "White"(which was always unleaded) would leave behind a sparkling clean engine that needed valve jobs about twice as often if the car was exclusively run on it.
For 1975 and later M/Y cars(US, first year of catalytic converters), most makers using cast iron engines went to a short term fix of induction hardening the valve seat. This worked, but the hardening was thin enough that it would be gone as soon as someone lapped the valves. Of course, the permanent fix is to machine out the valve seat area and fit a seat made of Stellite or similar material. I know cast iron engines are a dying breed, but presumably all have this from the factory now(and aluminum engine heads have always been had separate valve seats).