I had the same question so I called the Shell tech phone line. The fellow said that Defy used to have the boosted zinc level but no more. It is lowered to be consistent with the SN rating. Of all of the Quaker State/Pennzoil/Shell motor oils they sell, he recommended Rotella T4 for flat tappet engines. It has 1100 ppm of Zinc. If you are looking for conventional motor oils, there are the specialty oils like Joe Gibbs, Brad Penn, etc., and diesel truck oils like Rotella and others. If you are looking for synthetics, then Mobil 1 has several. The 15w-50 specifically says it is for flat tappets. The 0w-40 and the 5w-50 also have over 1000 ppm of zinc. Castrol Edge 5w-50 states it is for classic cars and older vehicles. So there are plenty of choices, but what was good in the past may not be in the future so it's always good to check.