Robin
The ZDDP has been removed, for the most part, with the last couple of specifications of gasoline engine rated oil. Here is a decent article on the service classification ratings.
http://www.pqiamerica.com/Labels.htm
The ZDDP was dropped in the last couple specifications of oil, and will require additives. One option is the high mileage oils, which generally have more zinc.
The diesel rated oils have plenty of ZDDP, and are a good alternative to buying those little $10 bottles of additive. It is not just the expense, but even finding the stuff.
Personally, I have been going over to the Shell Rotella as well, especially the synthetic arctic grades. I don't know how I would have survived this last winter without it.
Back in the 80s, the oils I was familiar with had both diesel and gas ratings to them (ie SAE SF-CD). Now they seem to have split the diesels away from the gas oils.
End of the day, one must do some homework on oils for the older engines and in some cases, use older specification oil for older engines. You can still buy 30 weight with good levels of ZDDP, and you can likely buy 30 weight without appropriate levels. It all comes down to the rating. Pretty much goes for all the weights and multi weights of oil.