Ok, I am a TOTAL dork, I was thinking about it last night and I dont think I was as clear as I could be about ZDDP. Let me try here. ZDDP is an extremely cheap dual purpose additive. It's only claim to fame is that it can make a very cheap base stock perform like a higher grade base stock under some very limited conditions. Chlorine and ZDDP both have the ability to enhance the camshaft protection of inferior oil base stocks. Both also have side effects that are not acceptable under at least some conditions, in at least some cars.
If you go by ZDDP alone, ANY current low SAPS/low ZDDP diesel oil is far "better" than the oil used a few years ago that suposedly prevented any problems.
In the case of camshaft lobe and flat tappet lifter protection, it serves no purpose until after the shear forces exceed the protection capability of the base oil. You are far better off selecting an oil that has passed industry standard test sequences specifically designed to measure the ability of an oil to protect flat tappet cam/lifters.
If one insists on hanging their hat on ZDDP (many do),understand that there are two different common types of ZDDP. Make sure you get the right one. Or, just stick to the industry standard ACEA oil service ratings that insure the type of protection you need.
ZDDP is used in vitually every snake oil on the market. GM EOS
http://www.gmpartsdirect.com/results.cfm?singlepart=1&partnumber=EOS is in my opinion a more credible supplement than prolong and does contain ZDDP. I have not checked but it is a safe bet that prolong also contains ZDDP. All snake oils seem to. So put ANY snake oil in ANY HD diesel rated oil andd it will have more ZDDP in it than the older gas engine oil we used a few years ago, because it already does without adding anything. SAPS (ZDDP) was first limited in gasoline engine oil starting with ILSAC GF-1 in 1992 and the limits became increasingly stringent with GF-2, GF-3 and now GF-4. API CJ still allows far more SAPS/ZDDP than ILSAC GF-3 did several years ago.
Prolong claims on their website that they don't use zinc.
http://www.prolong.com/A_ET.html The Z in ZDDP stands for zinc. But, it is probably not beyond any snake oil company to to claim to not use zinc when they do indeed use ZDDP and defend the position by stating that they meant no "elemental zinc" and did not intend their claim to mean that they don't use any zinc bearing compounds like ZDDP. I would be VERY surprised if there is no ZDDP in prolong. But, I would not be surpised by prolong with ZDDP failing to solve camshaft break-in problems. In fact.... The Gov came down on Prolong.... More marketing genius!
http://www.ftc.gov/opa/1999/09/prolong.shtm
An ACEA A3/B4 approved oil in an appropriate viscosity with the addition of GM EOS makes better sense. Again, I cant see a good reason to do it, and you are lookign at reduced cat life. there are no ZDDP limits on ACEA A3/B4 approved oil. As long as they are not also C1,C2,C3 approved they will still have the full complement of ZDDP in them. There is no such thing as an oil that is ILSAC approved AND ACEA A3/B3 approved (but some use misleading claims that sound like they meet both standards).
The fact that so many people are now spending considerable energy trying to find sources of ZDDP, while those same people are still ignoring the industry standard ACEA oil service ratings that include ASTM standardized test sequences that reliably measure the oils ability to protect specific types of camshafts/lifters against wear. Wives tails die hard.
The following all happened at about the same time:
1) Most flat tappet lifter manufacturing ended in the United States and was replaced by offshore manufacturing (mostly China)
2) Camshaft/lifter companies experiented with new lobe profiles and metalurgy. Their competitors followed suit.
3) The main flat tappet cam lobe wear test sequence was removed from the API/ILSAC oil service rating requirements
4) The SAPS limits in ILSAC approval dropped very slightly resulting in another incrimental reduction of the maximum ZDDP level allowed in ILSAC approved oil.
With all 4 of the above occuring at the same time, you would really need to be a camshaft/lifter manufacturer to blame all of the problem on ZDDP.
With all that in mind, for the Rover V8, if one REALLY wanted more ZDDP I would use an oil that is ACEA A3/B3, A3/B4 or API-CI4, CJ4 approved combined with the GM EOS. This combo WILL reduce the life of oxygen sensors and catalyts. And, frankly, I dont think it would yeild any better results.