Some updates: Just switched to Havoline ProDS Eco 5W-40 API SP Full Synthetic in my Suzuki SOHC F6A flat tappet turbocharged engine after running a thousand or so kilometers on Delo Gold Ultra 15W-40 API SL Mineral oil.
Not sure if the oil turns the darkest possible black within that time because of the high SAPS content (1.4% vs 0.9% for the ProDS) or because of the very high TBN cleaning out the internals and putting all the gunk in suspension. Perhaps both. My exhaust (no cat) smells more like fuel only now, rather than oil and fuel.
However, I'm having a great time so far having switched this engine over to a full synthetic oil. The engine sounds sprightly and I feel slightly less drag while accelerating in the 3-5k RPM power band on boost. I'm sure the oil is more resistant to coking, too, which means I don't have to idle very much after a bout of hard driving to let the turbo cool off.
Delo Gold Ultra has 1200ppm Zn and 1100ppm P, while the ProDS 5W-40 has 890ppm Zn and 770ppm P. I figure that the benefits of using a good quality, VHVI base oil (I'm assuming) formula with a 168 VI (vs. 137 for the Delo) and a much lower cold viscosity (89.1 cSt vs 115 cSt) and a stout 40-wt hot viscosity (13.8 cSt) outweigh the disadvantages of not having "enough zinc/ZDDP." My valve spring pressures are probably not that high, and oil formulation has come a long way since the 40s when ZDDP was introduced, especially with anti-wear compounds and detergency in mind.
Even if one looks up the 10W-30 API SF specification for the now-obsolete oil that the manufacturer recommends for this engine, they didn't have nearly as much zinc and phosphorus needed to meet specification as the modern API SP oil does. This 5W oil is only a little bit less viscous at 40°C than Havoline Formula 10W-30 API SN mineral oil anyway.
A large part of wear occurs on cold start, and I notice my oil pressure light going off almost an entire second sooner than it did with the Delo.
Will report back with more observations. By the way, Shell's new Helix Ultra 5W-40 API SP oil is now finally on the local market. GTL base, just like the Pennzoil I've always run, 170 VI, 12.8 cSt hot viscosity, 1100ppm Zn and 925ppm P - if the formulation is the same as it is in the European market.