I once saw a yellow Rislone bottle. Part of the on bottle advertising called itslef "The ZDDP additive." STP isnt the way to go anymore.
Upon further research i discovered that adding about 1/8th the bottle at each OC was all the extra ZDDP you would need.
What app are you looking to use it in?
Why do you need a ZDDP additive? What is this engine?
A little extra bit of ZDDP is often a good idea. It reduces engine wear at a very slight cost of shortening Catalytic converter life. IMO zinc levels are below optimal for just about all modern oils. If you're going to have 2 cats per life of the car and boost the zddp at reasonable levels the latter isn't an issue IMO.
Back to the OP's question, there are affordable sources of zinc that cost about a dollar an oil change. I use about 1.3 oz of Lucas TB Zinc add'v. The bottle is maybe a bit over ten bucks but lasts a dozen oil changes (cost at about a buck an oil change); it is meant as a break in add'v at full dosage the zinc levels are astronomical (according to a voa), but at under 1.5 oz it serves as a zddp level booster (upping the level by around 200 to 300 ppms if i remember correctly).
there are a few other options; i would search for VOA postings here and pencil out a reasonable dosage.
I use ZDDPlus, only I add 1/3 to 1/2 a bottle (depending on crankcase capacity)-boosts it around 300 ppm. If you used the entire bottle in a 5 qt capacity system, you are looking at typically 1500 ppm of Zn/Phos, which might be a little more than needed
Yes, but the reduction in the Zinc dialkyldithiophosphate is partly replaced with other ingredients that do a good job and protect modern catcons but cost more. In any case, you can simply use a dual-rated diesel and gasoline engine oil to get all the ZDDP (not just zinc) that your engine needs. A 10W-30, or 5W-40 in winter, or maybe 15W-40 in summer works great. The brand is your choice--all are about the same...Chevron, Shell, Valvoline, etc., etc.
Some of the 'cheaper' additives are Zinc replacements and don't contain any zinc....I use zddplus and zddpmax. About 7 bucks for a full shot for 5 quarts of crankcase oil, to get to around 1600ppm. For flat tappet pre 1990 cars and motorcycle oils.