The Feds have required oil companies to lower the ZDDP levels to make catalytic converters last longer. What used to be good levels of ZDDP of our favorite oil brand may not be true today, even the diesel oils. So you would need to confirm the MOST recent levels of a manufacturer's ZDDP levels before you could state that it will meet your needs.
The newer engines have roller lifters so the camshafts can still live with the lower levels of ZDDP. (that can make the case for the Ranger roller cam in our 2300's) Our cars have flat tappet camshafts and require higher levels of ZDDP.
The good news is that newer cars today specify thinner oil weights that we don't normally use. So those thinner grades (like 0W-30, 5W-30, 5W-20 etc) are the oils with the lowest levels of ZDDP. Automakers specify these thinner oils for improved gas mileage and good lubrication during cold start.
For our older cars we should not use these thin oil grades. We have larger clearances. So the oil companies can maintain a slightly higher level of ZDDP for oils not specified by auto manufacturers for newer cars. However if the ZDDP gets too high they need to specifiy the oil as for "offroad use only".
ZDDP helps when you have high valve spring pressures pushing a flat lifter firmly against the camshaft. A car with heavy racing springs needs higher levels of ZDDP for protection.
Now too much ZDDP can be bad! It can wear out your bearing! So that is why I do not recommend additives. ZDDP & phosphorus levels are measured in ppm. Yes that's "parts per million"! Are you a chemist to get the mixture just perfect?? The ZDDP already in your oil plus the additive???
I like Mobil 1 oils. I have driven my 1600 Pinto over 80,000 miles on it and the engine looks like new inside. My old Audi 5000 the same. These cars are summer and winter driven.
Here is the link for the ZDDP levels:
http://www.mobiloil.com/USA-English/MotorOil/Files/Mobil_1_Product_Guide.pdf I recommend using the following grades with no additives for the following conditions:
Mobil-1 15W-50 for temperatures above 50 degrees F. It has the perfect ZDDP levels and the right viscocity for both stock and light performance older cars.
Mobil-1 10W40 high mileage for 25-60 degrees F. ZDDP levels are OK for a broke in stock engine.
Mobil-1 0W-40 European for below 25 degrees F or cold starts below 45 degrees F if the car sits a lot. ZDDP levels OK for a broke in stock engine
Mobil-1 0W-50 racing oil for high performance engines. Very high ZDDP levels, film protection, and for a broad temperature range.
The only time I would add ZDDP additives is when you rebuild a engine and are doing the break-in.