ZDDP plates the bearing surfaces so that if the oil film breaks down either through lack of oil at a cold start or under extreme high temperature and pressure the ZDDP acts as a sacrificial layer preventing metal to metal contact. But here's the rub (pun intended) this protective layer wears off and it is temperature activated so that it doesn't get deposited again until the oil gets hot (80 Deg C is one figure I've seen).
And if you commute daily like me and the oil doesn't ever get to 80 C ?
This is why I've been banging on about HTHS which is what prevents the oil film breaking down in the first place and it's also why I worry more about how thin the oil is for fast cold start circulation than I do about how thick it is when it's hot.
Some info about HTHS which is the viscosity at 150 deg C:
SAE is the Oil viscosity standard the entire industry goes by. This standard is called J300.
A 30 weight oil is required to have a minimum HTHS of 2.9
40 weight oils are split in this regard:
0w-40, 5w-40, and 10w-40 grades of motor oil are required to have a minimum HTHS of 2.9
15w-40, 20w-40, 25w-40, and straight 40 grades of motor oil are required to have a minimum HTHS of 3.7
A 50 weight oil is required to have a minimum HTHS of 3.7
All the above HTHS numbers are MINIMUMS. Some motor oils will have a HTHS at the minimum level, and others will have a HTHS which is much higher.
HTHS is the main specification to determine the high temperature protection that a motor oil gives. This is the protection that a thin film of oils provides which prevents engine parts from rubbing against each other and wearing out. If choosing a thinner oil like 10W40 you really want to check the oil has higher than the minimum spec. You can also see from the minimum requirements that if you can't find 10W40 with a higher HTHS or the info is just not available then 15W40 is a safer bet if you still want something thinner than 20W50.
Getting back to ZDDP
Because of concerns that ZDDP poisons Cat converters the most recent API SM oil grades have reduced the maximum allowable ZDDP content to 0.08% (800 PPM, Parts per Million) from 0.10%, 1000 PPM.
The 0.10% limit had been in place for approx. 15 years in the USA and since 1986 in Australia. The concern about ZDDP levels is relatively recent and we strive for the old SG oil spec with 1200PPM, yet main stream auto oils have not had that level for a very long time.
No question that ZDDP is a good thing (if your oil gets hot) but you can have too much of a good thing. High levels of ZZDP cause corrosion and upset the detergent qualities of the oil so I wouldn't recommend trying to out smart the oil companies by DIY addition of ZDDP as an oil additive.
