Tests have shown that the amount of wear on a cam decreases as the amount of ZDDP is increased, up to around 1,400 parts per million (PPM) zinc. Above that level, the wear will actually increase due to zinc pitting. So all automotive oils have contained around 1,400 PPM of zinc additives for many years.

If you need oil with extra ZDDP, the best procedure is to use oil that already contains the additive mixed in the proper dosage.

Too much ZDDP can do almost as much harm than not enough.

High levels of ZDDP can begin clogging the lubricant-carrying crevices in the cylinder wall, which can then cause the engine to burn oil.

Total Seal’s Keith Jones told us that a few customers who had been using race oil in high-output street engines began to burn oil beyond what they considered normal.

Jones told us his solution was to have the customers try an oil change using a high-detergent diesel oil for roughly 100 to 200 miles. The reason behind this is that detergents tend to strip away ZDDP from the valleys or crevices created in the cylinder wall during the honing process. By removing the excess ZDDP by running a high-detergent oil, the engine’s excess oil use returned to normal. This also illustrates that properly blended performance oils offer a careful balance between detergent levels and ZDDP. Adding ZDDP to the oil changes that careful balance which likely will not produce the desired effect.

Essentially, too much ZDDP can be just as bad as too little.

I took this question to Lake Speed Jr. who is a tribologist (an oil engineer) with Driven Racing Oil and he offered a few more points.

“Mixing an additive into motor oil can cause adverse chemical reactions. ZDDP is acidic in nature and motor oils that contain detergents are basic in nature. As a result, mixing ZDDP into high-detergent oil can cause an acid/base reaction,” Speed Jr. said.
(note from HP - all street oils are detergent in nature. This is where real racing oils come into play: low levels or lack of detergents. Which is also why many racing oils have really high levels of ZDDP. It's not recommended to use a racing oil on the street...and no...Valvoline VR-1 is NOT a true low detergent racing oil.)

As an example, Speed offered the rather dramatic case of what happens when you mix baking soda and vinegar. If you’ve not witnessed this first-hand, the result is a foaming mess. Mixing ZDDP additive with your oil won’t be as dramatic as this vinegar-soda example, but unless you perform a chemical analysis of the oil after mixing, you really won’t know.

“Motor oils have a blend order that keeps reactive additives in the proper order of addition to prevent adverse chemical reactions,” Speed Jr. said. “By introducing an additive to motor oil, the blend order has been altered, and that increases the chance of an adverse chemical reaction.”

As a follow-up, Speed Jr. said that adding ZDDP alters the oil’s actual chemistry.

“Adding ZDDP to motor oil lowers the base number and reduces the service life of the oil,” he said.

The base number to which Speed Jr. is referring is a rating of the oil’s ability to resist corrosion by neutralizing acids that naturally form as a result of combustion. This is especially troublesome for engines that do not run long enough to exceed 212 degrees F of oil temperature. This is the point where the water (and acid) would boil off as a vapor and are eliminated by the PCV system.

This change in the engine oil’s base number means that well-intentioned amateur chemists “splash-blending” ZDDP with engine oil will likely reduce the oil change interval by making the oil more acidic. We’ve seen engines that have been stored with old, acidic oil. The bearings were terribly damaged with deep corrosion pits. So this is another factor to consider.

As you can see, there are several reasons why adding a ZDDP additive to engine oil is a bad idea.

Frankly, this extends to an additive product of any kind for many of the same reasons. The point to remember is that motor oil is a very complex mix of chemicals that are carefully blended to work together. It’s best not to mess with that success.

So if mixing ZDDP isn’t a suitable option, we’re back to using engine oil specifically intended for flat-tappet performance engines not fitted with catalytic converters.