Just about any motor oil is at least 25% additives. Subaru’s warning makes no sense unless they specify WHICH additives to avoid.
Forget about special deals such as those offered to you. Synthetic oils wear better and are preferred to regular mineral oils.
No one can really offer an engine warrantee based on the oil you use.
So, go ahead and use a high quality synthetic but expect just good service and performance.
Both my cars have Mobil 1, a high quality synthetic, but achieving high mileage will depend on my driving style mostly!