Because the EPA wants a minimum 100,000-mile lifespan from catalytic converters, they need a fighting chance at survival. Zinc in the oil undermines that survival. That's why both the automakers and Washington decided zinc had to be eliminated from engine oil.
Without zinc in the oil, wear for vintage engines with flat tappet cams increases exponentially. In fact, it is alarming how quickly it happens and how much damage it does. Zinc is crucial not just for cold start-up, but extreme conditions that make heavy-duty engine oils necessary for reliable operation. There's no magic in heavy-duty engine oil, just higher levels of ZDDP to help reduce wear.
The first thing you want to know about engine oil is if it has an "SM" rating, which indicates greatly reduced or zero zinc levels, which makes it harmful to your classic Mustang's engine. Do not use engine oil with the "SM" rating. Or, if you're going to use engine oil with the "SM" rating, use a zinc additive that will maintain proper ZDDP levels. Front and center for this issue is California, which wants all zinc eliminated from engine oil. Regardless of what government and industry mandate for engine oil, it is up to you to ensure sufficient zinc levels are maintained when you change or add oil.
ZDDP is crucial to engine wear and break-in issues because so many things need to happen when you fire an engine for the first time. When you fire a vintage engine with flat tappets for the first time and run it at 2,500 rpm for 30 minutes, you are work-hardening the cam lobes to ensure long life. Cam lobes not only move the lifters, pushrods, and valves, they also spin the lifters in their bores for proper function. The lifter and lobe must have sufficient traction for spinning to happen. This is why you don't want to run synthetic oil or friction-reducing additive during break-in. ZDDP helps lifter/lobe traction. It also works into your engine's hardest working parts.
Marvin McAfee of MCE Engines...
Marvin McAfee of MCE Engines suggests using a powerful magnet at the pick-up screen to keep ferrous metals out of the pump. Magnetic drain plugs are also an excellent idea because you can see ferrous metals, if any, during oil changes.
Which type of engine oil should...
Which type of engine oil should you use and what viscosity? Freshly built engines should get a conventional, non-detergent, heavy-duty SAE 30 weight oil for proper break-in. Once the engine has passed 500-1,000 miles, you can run Mobil 1 10W30 or 10W40 synthetic along with a ZDDP additive. If synthetic is beyond your budget, choose Castrol, Valvoline, or Mobil 10W30 or 10W40. High-mileage engines that have had a diet of conventional oil need to stay with conventional oil, either 10W40 or 20W50.
Read carefully what's on the...
Read carefully what's on the bottle. A "newer vehicle formula" means the oil probably doesn't include zinc. If you see "SM," that means low zinc or no zinc.
Conventional engine oils are...
Conventional engine oils are the right choice for break-in and for higher mileage engines. Viscosity should be 10W40 or 20W50 for high-mileage engines. Aside from ZDDP, your engine oil doesn't need any other kind of additive.
ZDDPlus from Navone Engineering...
ZDDPlus from Navone Engineering (www.zddplus.com) is a very effective zinc-based oil additive that should be used right from the start with a new engine. It is also the most effective oil additive you can buy for older engines because it slows wear. You can get a discount when you buy a six-pack.