Oil viscosity

Thanks for that link! These 2 paragraphs in the above discussion explain that this need for ZDDP is indeed spring pressure and RPM dependent:

"There is on the other hand, research that concludes that the minimum ZDDP requirement is directly related to the lifter foot pressure. In one SAE paper it is reported that: “at a ZDP level corresponding to 0.02% phosphorus, scuffing occurred at 200 pounds lifter load, while it required 240 and 480 pounds lifter load for oils containing 0.04 and 0.06% phosphorus, respectively, to initiate scuffing. At 0.08% phosphorus concentration, no scuffing occurred up to 600 pounds lifter load, the test hardware limit. Scuffing occurred at 350 pounds lifter load with no ZDP (0% phosphorus).”

The older engines and high-performance engines we are concerned about may have lifter foot pressures several times that of a low-performance engine such as those used in the Sequence III tests, and their wear characteristics are not predicted by common Sequence III testing methodology. An additional factor is the dynamic load at the lifter foot. Sequence III engines run at 3600 RPM maximum during the test. Most high-performance TR engines are regularly run to 6000 RPM. The inertial contribution to the lifter foot pressure increases as the square of the increase in RPM. This means that the inertial load at 6000 RPM is 2.67 times it’s value at 3600 RPM."

And this is interesting and points to starting with a 1400-1500 PPM ZDDP level and changing oils regularly:

"We have never been able to find the results of these tests on older engines. We would need to study those reports to see exactly which engine types and cam/follower types were involved. The fact is that all API test sequences we have studied use non-performance engines with low spring pressures, indeed in the Sequence IIIG test, the static lifter load is 350 pounds . Many high-performance engines have as much as 500 pounds or more of lifter foot pressure. Referring to the Bennet data, this would indicate that in order to keep from scuffing, a ZDP level giving a .065 % minimum phosphorus level would need to be ensured. If one considers that fact that the ZDDP level constantly drops from the initial level as a vehicle is driven, a safety margin above that is advisable. This means that if one wishes to maintain .065% minimum phosphorus, more than that must be present in the initial fill. Our calculations estimate that if you start with a ZDDP level which gives .14% phosphorus, after 2000-3000 miles, the actual ZDDP remaining active has dropped to the point where there is just enough protection."