The QuestionWhich oil should I use in my classic car? It’s incredibly important to ask this question. Why? The reduced level of zinc dialkyldithiophosphate (known as ZDDP, ZDP or ZnDTP) in modern motor oil has been linked to increasing numbers of tappet and camshaft failures in vintage engines.What Exactly Is the Problem?The cam/tappet failure problems often begin with a freshly rebuilt engine that starts making expensive-sounding noises. Inspection might reveal that the bottom of one or more tappets is gone. Instead of a smooth, machined surface, the face of the tappet will look like the surface of the moon. If the problem is the camshaft, it will exhibit one or more worn lobes.Just one failed tappet or cam lobe will create a problem, as the damage results from direct metal-to-metal contact. With metal debris in the sump, there is no choice but to tear down and rebuild the engine. Choosing an assembly lube and motor oil is critical in preventing this metal-to-metal contact.Corrosion, which occurs over time when classics are not driven, is another serious issue. Normal motor oil is designed to lubricate, not to coat or protect metal surfaces from corrosion. All oil absorbs moisture from the atmosphere. Running the engine will eliminate this moisture, but leaving a car to sit for extended periods of time will lead to corrosion. Using an oil product that forms a clinging protective film on the exposed metal parts can minimize this problem. If the oil contains special corrosion inhibitors, all the better.Repair shops specializing in British cars have been dealing with these issues for years, and most have developed a combination of parts, machine work, engine prep and lubricants to reduce these problems. Many shops cite assembly lube, oil and the amount of ZDDP in the oil as major concerns.What Is ZDDP?Zinc dialkyldithiophosphate is an oil supplement that has served as the primary extreme pressure (EP) ingredient in all quality motor oils for the past 70 years—until recently. What does it do? When exposed to heat and pressure, ZDDP forms a protective film on metal surfaces that prevents parts (cam lobes and tappets, for example) from making metal-to-metal contact.Why Do I Suddenly Need ZDDP?ZDDP has been phased out because it damages catalytic converters. Small amounts of zinc and phosphorus in the ZDDP coat the catalytic material, reducing the effective life of the converter. The ZDDP level in motor oil was reduced from 0.15 to 0.12 percent (1,500 to 1,200 PPM) in 1993, and further reduced from 0.08 to 0.06 percent (800 to 600 PPM) in API SM-grade oil in 2004. But is this level enough for an older engine, especially when it isn’t run frequently? And is it enough to protect the cam and lifters in a freshly rebuilt older engine during the critical break-in period?The experience of hundreds of professional engine rebuilders, cam manufacturers and restorers indicates the mandated ZDDP level is not enough. The Engine Builders Association concluded that 75 percent of reported cam/tappet failures were due to the reduction in ZDDP. Association Technical Bulletin 2333R (November 2007) says current engine oils used by engine manufacturers in new car production should not be used for initial flat tappet/camshaft break-in. It recommends adding additional zinc for camshaft and lifter break-in. Most cam manufacturers also have specific instructions regarding assembly lube and break-in oil, citing cam/ tappet failures.So What Should I Do?The following guidelines can help you prevent cam/tappet failure and protect your engine.Initial Break-In Period (First 30 minutes)Use oil with ZDDP at 0.14 to 0.15 percent by weight (1,400 to 1,500 PPM) to provide the additional protection needed to maximize the chances of a successful cam/tappet break-in.First 500 Miles After Initial Break-InAfter that initial 20- to 30-minute break-in period, change the oil and oil filter. The oil you run after break-in will not need as much ZDDP; 0.10 to 0.12 percent ZDDP will provide protection without risking chemical corrosion.Second 500 Miles After Initial Break-InAfter the first 500 miles, change the oil and filter again, using oil with the same ZDDP level, 0.10 to 0.12 percent.After the First 1,000 Miles (Car Driven Infrequently)If you don’t drive your car once a week for 30 minutes or more with the oil between 170 and 200 degrees, consider using oil formulated specifically for classic cars. This oil has a mixture of additives designed to deal with the moisture, corrosion and acids in engines that sit for extended periods of time. Change your oil every 3,000 miles or every six months, whichever comes first. If you live in an area with high humidity, change the oil and filter four times a year.After the First 1,000 Miles (Car Driven Frequently)If you drive your car once a week for 30 minutes or more with the oil between 170 and 200 degrees, you have more options. Driving the car frequently will minimize the amount of acid, water and water vapor in the crankcase, and that will limit the corrosion and subsequent pitting of the cam lobes and lifters. Using 20W-50 API SM oil with 0.08 percent ZDDP can be fine, but if you are more conservative, a ZDDP level of 0.10 to 0.12 percent will provide additional protection.