OK, a few clarifications.
1. Catalytic Converters don't like ZDDP. That is why the EPA forced the reduction of ZDDP in today's engine oils.
2. "Slickness" is not the problem. ZDDP provides protection against the "wiping" action of the camshaft against the lifter. This is the failure point in engines with fairly high valve spring pressures and non-roller lifter valvetrains, if the engine does not have adequate levels of ZDDP in the oil. Many older engines were not performance designs and used low valve spring pressures and as such, even today, will not reflect valve train problems using lesser amounts of ZDDP in the engine oil.
3. While synthetic oils have definite advantages, they are "slicker" and as a result will find ways to leak in older technology engines. Individuals may have Studebaker engines that do not leak with synthetic oils and that is good, but engines not designed for synthetic oils, will, as a result of the design, have a greater tendency to leak oil when the are filled with synthetic oils.