I'm glad you asked this question, I've been pouring over forums trying to figure out what the best option is these days (I too am a new Pantera owner).
That said, prepare for a long answer based on the research I've been doing the last couple of weeks - the Web is a very confusing place.
I've read a number of comments from people - many claiming to be engine builders - saying that if the bottle has an API SM rating, then the ZDDP level is about 1/2 to 1/3 of what it was in the '70s. So, armed with that information and I've gone up and down the aisles at auto shops, gas stations and even (gasp) Wally World looking for something without the SM rating. I finally found some Quaker State oil that's being marketed as being designed for high mileage older model cars, and it didn't have the 'SM' designation (I thought I'd finally found the oil I needed), but on further investigation, found a thread somewhere that said QS didn't go through the official channels and pay to use the API SM stamp, so they had to stop putting it on that particular type of oil (so now I don't know if the oil has higher ZDDP, or is just a cheaper modern oil without a stamp on the label).
Then there's the threads that say you don't need the ZDDP in classic engines - if you look, you'll find them and the flame wars back and forth.
Then there are the conflicting threads from companies that market ZDDP additives, and people saying don't put in additives, it's better to use straight oil from a manufacturer who puts together the right balance in the first place.
So, after reading lots of opinions from people on Ford sites, Chev sites, and Mopar sites, I was planning on asking this very question here as I have greater trust in people I know aren't here to market one product over another. Hence, When someone like George P. (KFH) says you need ZDDP, I'm inclined to listen.
Unfortunately, up here in the great white north, I haven't found Amsoil products, so I've been pouring over Used Oil Analysis reports at this Used Oil Forum and looking in particular at the Zinc and Phosphorus numbers (the ZDDP).
The most common advice I've seen from the classic car sites is that you want the zinc to be around 1200ppm in the oil, and the currently available (where I am) oil that is hitting that mark is Shell Rotella T 5W-40 (UOA reports I've found on it regularly report Zinc and Phosphorus well above 1000ppm (often up around 1300 for zinc), while many other modern oils are down around 600-700ppm)
Mobil 1 posts it's Phosphorus & Zinc ppm table here - Mobil 1 Engine Oils - however, most if not all of these are fully synthetic, so based on Comp2's reply, you may want to avoid these.
As for Castrol, if you look at this Q&A on their site (see #2) the author admits that ZDDP is needed, but then when he talks about Castrol's response, he uses the future tense (meaning that they haven't got anything yet).
I'm still digging to try and find a UOA on the Amsoil 20W50 - the 5W30 from Amsoil doesn't appear to be quite as good as the Rotella T based on the numbers I've found, but it seems to still get very respectable results compared to other oils - the Amsoil 20W50 could very well be much better, I just haven't found it yet (and it has the CFH seal of approval, so maybe I need to look harder
) Redline and Brad Penn oils look good too.One word of advice is to look at the source of everything you read, many forums are backed by oil companies, additive companies, or even sales reps from one or the other (even the one where I'm reading the UOA reports is rumoured to be hosted/sponsored by a rep from a specialty oil company) .
Good luck.
Whew, I hope that helped!
Note: I'm not an oil expert, I'm just a guy like you that wants what's best for my cat and looking to the gurus on this forum to set me straight if anything I've found in my research is flawed.