One thing I noticed when running dino oil in my 65 was a lot of chatter on start-up. I attributed that mostly to the cheap filters I was using, so switched to Wix. That helped some, but I still got some chatter. Well, with synthetic, I have absolutely no chatter at start-up. That's just one indication of the superiority of synthetic oil. The other is that my engine seems to run a lot smoother, with what feels like more power, and I attribute that to the extra lubrication that synthetic offers. As far as the cost? It's about the same. Do some research, and you will discover that you can run synthetic at least twice as far as dino. Now, you may have to change the filter between oil changes, but that's peanuts.
Yeah oil sludge is just a matter of neglect. You don't need to change the oil at 3K intervals though. That's just oil companies trying to make a buck. Drive your engine 3K miles with your diesel oil, then take it to have it tested. You will be surprised at how much good oil you are throwing away. As far as condensation is concerned. Yeah oil and water don't mix. I change my oil right after the winter season because that is when condensation usually occurs in the oil pan. And, I try to drive my Mustangs at least once a week to burn off any condensation. My truck gets driven daily. As far as the phospherous and zinc content in synthetic oil, it is a misunderstanding if you think synthetic oils don't contain them. Hell, even Mobil1 contains them, and you can get that at Walmart. Here are some other examples.
AMO 10W-40 Synthetic Premium Protection
ARO 20W-50 Synthetic Premium Protection
HDD Series 3000 Synthetic 5W-30 Diesel Oil
TRO 20W-50 Synthetic Racing Oil
AHR SAE 60 Synthetic Racing Oil