Check your knock sensor wires for rodent damage.
Toyota's must taste pretty good to these critters.
There are also two knock sensors, passenger and driver, on some...
If you have to, install an oil cooler...
When was the last time you had an emissions inspection? Check the NOx numbers on your test papers. Are the numbers pushing the test "fail" limit?
The symptoms you describe kind of point to the EGR system either not working or EGR passages plugged or patially plugged.
The EGR system is designed to lower combustion chamber temps (this lowers the NOx numbers). Higher combustion chamber temps would cause elevated NOx numbers and the pinging you describe.
pinging cant be good for any engine, so my question is mid grade or premium worth the added cost vs effects on engine. I'm trying for the million mile club :}
Very light engine pinging is ok for an engine. Some engines are designed to do so to optimize efficiency. Heavy constant pinging or knocking is not. The ECM on a modern engine will detect the latter, and retard timing accordingly. But mileage and combustion temps will suffer.
Use the recommended octane. The % cost difference is often less than the % mileage loss of using lower octane; miles per $, the higher octane, if required, is the better value.
If you have excessive pinging on recommended octane, try a combustion chamber cleaner (Techron), or change gas stations. On some engines, resetting the ECM to clear drive data can provide a fresh start.
FWIW, gas loses about 1 octane point/month. The fresher the fuel, the better.
I have the same problem with my G35. It has no EGR. Because it is a 6mt, premium is required. The car has pinged a lot since new, but it has gotten worse with miles. It even pings before the engine is up to temp.
It pings at low speeds, but is worst around 4,500rpm.
My Taurus pinged horribly for 4 or 5 years. It got to the point that I would just let it ping bad hoping it'd blow a hole in one of the pistons and I could get warrantied a new engine to fix it.
Spent a LOT of money trying to stop it from pinging.
Turns out, all those years, it was a bad EGR sensor causing it to always run seriously lean.
I check the junkyard for the car when I go. If I ever come across it, I'm going to pull the front head to see how badly damaged the pistons were.