Yes they would have to prove it. And that assumes the warranty is voided by the oil grade, I doubt your warranty booklet or owner's manual states that grade selection is a criteria for continued warranty.
Also good luck differentiating a high HTHS 5W-30 from a 40-grade especially after use.
hmm I do see that but I read it as 5w40 only if 5w30 isn't available, however I do see the in hot weather comment. So much wiggle room in the manual, I wonder how much they (Subaru) would actually fight the use of 5w40. IIRC they would have to prove the oil weight caused the issue correct?
Besides oil brand, the hottest oil topic about Subaru’s is interpreting the manual. From what I’ve read, most dealer’s are fine with 5W-40 in a WRX/STI. Someone on the Outback forum recently had a discussion with SoA about 0W-20 which is recommended for the new 2.4T. After a lot of back and forth, and with that person mentioning the thicker viscosity in hot weather language, he finally got them to say a 30 grade would be fine.
I think any disputes in regards to oil viscosity during a warranty claim are rare. I go to my dealer for a la carte maintenance and they’ve gone to bat for me in the past with warranty claims. A lot of it has to do with the discussions between the dealer and the district Subaru rep.
Back when the Outback specced 5W-30, a former member and Amsoil dealer here went with other grades right away. I think he even used the 5W-30 diesel for an interval.