yes, the additive package does make a difference. In the middle 1970's Chevron changed the additive package without telling Cummins and it cost Cummins a huge amount of money because of sudden oil consumption issues. Before that issue Cummins used Chevron Delo 400 exclusively and after the oil consumption issue they changed to using Rotella T as the recommended oil. I think they also sued Chevron but I can't remember for sure.
Anyway, additive packages does make a big difference in oil performance and you have to be careful what oil you use for each application.
When I change the tranny and axle oils I will probably use the EP oil.