I'm not sure if it will ever cause an issue, but Rotella does not meet Subaru minimum requirementsI used Pennzoil Formula Shell Synthetic for my first oil change,paired with the OEM filter .But for my second oil change I'll be using Rotella Shell T6 5W-40 after reading a lot of good stuff about it.
I'd save all the receipts in case any warranty issues were to arise.
I know for one it isn't "energy conserving" which is a requirement, and I though it had to be API CN certified (could be wrong on that, cant pull up my manual on work computer) which rotella is not. I was going to use it because, like you, I've heard nothing but great reviews, but those were two things I noticed. Like I said though, they may not be that picky and you may never have a problemwhy not?
The oil weight is within spec, that's not an issue. I'm just curious about how picky they'll be. I would say the best thing to do is call the dealeryou would use for warranty work and get their input. Most major oil for gasoline cars say energy conserving. Rotella is designed for diesel which could be why it isn't energy conservingIf I remember correctly they do recommend 5w-30 synthetic SM/SN Energy Saving oil. Though they also mention 5w-40 synthetic can be substituted under severe conditions. I don't see a reason how Rotella T6 would void it. It's 5w-40 SM the only thing it's missing is Energy Saving which I haven't seen in any 5w-40 oils.
Given your location it interests me to hear what you run in winter. Would you be OK with a synthetic 5W-40 in winter? If so, you could simply run the same T6 year round. Otherwise there is M1 0W-40, but my motor did use more of that than the T6 5W-40. Either would be fine for me but the weather my car sees has no relationship to what yours goes through.I have seen the test results on the T-6 at 5000 miles and it still met spec. I'm past my warranty either way and will be making the switch this weekend.