Impressively, since that time the average vehicle age in the U.S. fleet climbed to 11 years. There are about 250 million gasoline fueled cars and light trucks on the U.S. road, and in November, Experian Automotive reported that almost 51 percent of them are model years 2000 to 2008. They have been fed a steady diet of low-ZDDP engine oils – and have seen no major catastrophes related to premature wear. It seems fair to say that engine wear is under control for the vast majority of U.S. cars.

Still, a small body of end users – restorers and drivers of high performance vehicles or collectible cars – demur. They want more ZDDP for their engines, many of which are designs dating back 30 years or more. They worry that the scant phosphorus in modern oils cant keep a collectible car in top condition, or stand up to hot-rod racing, or withstand the break-in regimen for a rebuild.

PCMOs used to be adequate to protect the cams in classic muscle cars, British cars, old motorcycles, vintage collector cars, small engines and many engines used in race cars where the rules prevent changing to a roller cam, one enthusiast pleaded in a recent exchange with LubesnGreases. Wouldnt a diesel engine oil or classic engine oil or racing oil be a better fit for these engines now?

This is a classic concern raised by many car buffs and the subject of many internet conversations as well as car clubs and restoration group meetings. In response, several lubricant industry experts have presented information to these groups.

One additive supplier technical manager, who spoke off the record, sees compelling arguments on many sides. In a talk during a National Corvette Restoration Society meeting, he noted that engine valve-spring tensions used to be much higher, metallurgy was not as well controlled, and surface finishes were not as smooth as today. On the other hand, in addition to higher ZDDP levels, most legacy engines also used SAE 20W-50 and much thicker oils than today. And while new U.S.-made vehicles have roller-follower cam lifters, many Japanese automakers still favor slider-follower valvetrains, which wear differently.

For classic-car applications, this source has been recommending SAE 15W-40 heavy-duty diesel motor oil. That provides 0.12 percent phosphorus, and the type of viscometrics on which the old engines were designed to run.

Another industry veteran, Bob Olree, spent years working in engine oil development at General Motors Research. Now retired, he recalls that GM found that 0.08 percent phosphorus eliminated many wear issues in the mid-1950s. However, ZDDP is a multifunctional additive and some oils had antioxidancy requirements that may have pushed total phosphorus concentrations above that level.

The only strong argument put forward by the oil and additive companies about the first proposals to limit phosphorus to 0.08 percent, Olree remarked, was that some oil formulations needed more ZDDP to pass earlier versions of the ASTM Sequence V engine test. The 2.3-liter Ford engine used in those tests had a finger-follower valvetrain that required an especially good antiwear package; it also had a design where harsh blow-by condensed on the valvetrain during the test.

How many classic and muscle car owners have encountered premature valve­train wear with 0.08% phosphorus oil? Few can actually say, I did, Olree contested. Most have only heard horror stories from a friend who knew someone who had the problem. Perhaps they didnt follow good rebuild practices; or used new cams but old lifters; or diverged from the critical break-in protocol of immediately running the engine at 2,500 to 3,000 rpm at zero load, he mused. The older cars on the road with broken-in cam and lifters dont need anything near the ZDDP level in current oils.

His fellow GM alumnus, consultant Mike McMillan, agrees that OEMs were highly concerned about wear as the industry shifted to lower-phosphorus oils. It was the hot-button issue during the introduction of ILSAC GF-3 engine oils in 2001. Several automakers wanted to limit phosphorus to 0.08 percent max, but GM in particular voiced doubts, McMillan said, so it remained at 0.10 percent. In addition, any oil with less than 0.08 percent phosphorus was required to pass the maximum wear requirements of the Sequence VE test.

By 2004, ILSAC members were more comfortable with the oils wear performance, and they moved in concert to limit phosphorus. For GF-4 they set the maximum at 0.08 percent, but also instituted a minimum of 0.06 percent, for a measure of security.

Oil marketers have come in for their share of questions about ZDDP content and its impact on engine life, and some have responded by seizing the market opportunity. Brands such as Valvoline, Amsoil, Royal Purple, Champion Brands, Brad Penn and others have fielded heavyweight racing oils and break-in oils promising high ZDDP levels.

Are these needed? Through a spokesperson, Amsoil said it has had no complaints about failed camshafts using its regular engine oil that could be the result of low ZDDP. It said a low-zinc oil, properly formulated using the right chemistry, can provide the protection needed for older flat tappets and higher valve-spring pressure. The typical off-the-shelf product, though, may not meet this need since both gasoline and diesel engine oils have lower ZDDP levels now than before. Especially for rebuilt engines, Amsoil recommends a higher ZDDP oil such as one formulated for diesel engines.

ZDDP is depleted over the oils service life, notes Don Smolenski, another GM retiree, now with Evonik Oil Additives. So to avoid bearing problems, drivers should closely follow their OEMs drain interval and viscosity grade recommendations. However, if there is particular concern about wear, one option is to be cautious and shorten the drain interval, he suggests.

But under no circumstance should GF-5 oil be used to break in a rebuilt engine, another source insists: I have talked with guys who did not reach 200 miles on high-dollar rebuilds before rounding off a couple of cam lobes, and others that suffered early cam failures, in terms of miles, on engines in classic cars.

Break-in oils used by rebuilders often have added amounts of antiwear protection, in addition to ZDDP. Chrysler, for example, used to add a sulfur-containing compound to its engines as a part of the factory-fill oil. But the oil is just one factor in a successful rebuild, says Bob Olree, and there are many others that can doom the cam and lifter break-in process:

1. Quality of the lifters, materials, heat treating and grinding.

2. Quality of the cams, materials, profile, lobe positioning, heat treating, grinding and phosphating.

3. Initial break-in operating conditions.

4. Lifter bore tilt and positioning in the block.

Olree puts oil quality later, after the above issues.

Crane Cams, which supplies parts for rebuilders, stresses that new lifters should always be installed with a new flat-tappet camshaft before break-in. The first 20 minutes of break-in are the most critical, it says, and for this it suggests oils such as Brad Penn, Maxima, a racing oil or similar specialty. Do not use API rated SL or SM oil, it strongly warns. (See Installation Instructions 548E at www.cranecams.com.)

Has the industry reached the end-point in ZDDP? Can it reasonably go any lower in passenger car engine oil formulations? Its worth noting that as long ago as the 1970s, oils containing less than 0.06 percent phosphorus were being field tested in mixed fleet operations – without any wear problems being uncovered.

Given that history, and tightening limits for vehicle emissions, it seems likely that another incremental drop in phosphorus is possible if not probable. So this debate is far from over.

Clarification: In Part 1 of this article, when citing the insights into cam and lifter wear gained through engine tests, Evoniks Don Smolenski was speaking of the current Sequence IVA test (ASTM D6981), not the earlier Chrysler procedure.