Well said.
I think the reason Ford went to roller lifters in cars was a compounding effect from wanting to improve fuel economy without sacrificing power or longevity. Moving away from carburetors to EFI was the prime ticket for accomplishing this, but so were changes in the valvetrain. Roller lifters can tolerate more aggressive ramps with a more rounded lobe which allows them to shorten the duration without sacrificing overall flow area under the curve. They could build more cylinder pressure, increasing torque (especially down low) and improving fuel economy.
There's some stock flat tappet engines that do still need more ZDDP in stock form, particularly Ford and Chevy big blocks due to the heavy valvetrain, high rocker ratio, big valves, and small lifter diameter. In an OHV (central cam) V engine with rockers, the rocker arm multiplies the force applied across it. The highest load is seen on the exhaust side when attempting to open the exhaust valve. This is because the valvetrain has to overcome residual combustion pressure in the cylinder to open the exhaust valve.
Let's take a stock 454ci BBC with oval port "290" heads which have 2.06" / 1.72" valves. Calculating exhaust valve surface area...
(1.72" / 2)^2 x π = 2.32 in^2 valve surface area
Cylinder pressure, with a stock engine and cam, is typically ~120 psi at the time the exhaust valve opens. This is multiplied by the valve surface area to get the load seen by the valve.
2.32 in^2 x 120 psi = 278 lbs
So there's 278 lbs seen by the valve against the residual cylinder pressure. The valvetrain must overcome this in order to open the valve, but it must also overcome the spring seat pressure which is ~90 lbs.
278 lbs + 90 lbs = 368 lbs
Since the rocker arm acts a lever, the load on the valve side is multiplied across the rocker ratio to the lifter side. The stock rocker arms for a BBC have a ratio of 1.7:1. Thus...
368 lbs x 1.7 = 626 lbs
This is the load seen by the pushrod and lifter the moment the lifter starts to climb the lobe. Inertia adds more on top of this. BBC has 0.842" diameter lifters which isn't a lot of contact area with the lobe so the force at that interface is immense. Also remember that the flat tappet lifters in these engines are constantly rotating. The lobe has an angle to it that spins the lifter. If the lifter stops spinning, it will dig into the lobe, destroying both the cam and the lifter. ZDDP provides enough cushion to keep that from happening.
By comparison, a 302ci SBF with larger diameter lifters (0.874"), smaller exhaust valves (1.60"), and lower rocker ratio (1.6:1) sees 530 lbs at the lifter. A 350ci SBC with yet smaller exhaust valves (1.50") and lower rocker ratio (1.5:1) sees 454 lbs.
For something like a Honda 4 cylinder OHC engine with the direct lobe to tappet setup, there's no rocker ratio to multiply the load and a lot less inertia thus the load seen at the lobe is significantly less. This is especially true of 4v pentroof heads where you have 2 smaller exhaust valves reducing the load to respective lobes. Thus, 600-800 ppm is perfectly fine even with radical cams. You'll never exceed what even a stock SBC or SBF sees. It's the same with flat tappet cams in small engines.
