If your Buick has the 3800 VIN C engine, you should be fine running 5w30 all year.
My '89 Olds Delta 88 (3800 VIN C) specified 5w30 as the preferred fill and ran great on 5w30, right up to the day I junked it, this past June, at 269,000 miles, because of severe rust.
I'm not aware of any...