The 5w-40 I see that advance has is the same price for gas or diesel, but they are 1qt not 5. And I probably wouldn't risk it, the 10$ saved isn't worth the damage that could be caused
Castrol 5w 40 at advance is 8.19 a quart. The 4 quart diesel synthetic is like 24 for a 1 gal jug. The diesel oil is cheaper but def not worth the savings as said.
Basically the additive pack, it has a lot more zinc in it. There's more to it than that but I can't remember the complete differences. I've known people to put rotella in their fsi and get an analysis, their sample was said to have had plenty of additive left and the breakdown wasnt significant. This was on 6k miles and they were advised to try 7500 miles next.