"There's a guy with 385,000 miles on a 164 running Castrol"
Remember, he was running the old Castrol, not the new stuff. They do have a new synthetic for older flat tappet cars, but I think it is pretty pricey if the car uses oil, as does my 91S, ~500 miles per QT. I use the Valvoline VR-1 racing oil in it, as it costs half as much, has the proper amounts of the good antiwear additives.
For the Milano and the 94LS, both of which use no oil between changes, I do use the much more expensive Mobil 1 15W-50 (with longer times between changes), which also has the proper amount of the same antiwear additives.