I use Mobil 1 15w-50 on all 3 of my diesels. It works great. I change the oil AND filter, every 4,000 miles. Supposedly they say it lasts 25,000 miles, but there is no way I am going to leave the same oil in my car for that long.
As long as you use a good filter (I use OE MB filters) and a good motor oil Mobil 1 synthetic (supposedly the best in the industry). The only downside is the cost. At almost $5.00 per quart and 8 quarts, plus filter, it costs more than your typical oil change.
I kind of wondered about the (owners manual-7,000 miles vs 3,000 miles-jiffy lube). I think its better to change it more frequenty than wait 7,000 miles. Also your driving habits, driving conditions, etc.
On the car you are restoring, do you plan on doing a complete engine overhaul? Because I heard its not good to use synthetics on a older car with a lot of miles. And you are right about how the additives wear out, etc.