WARNING: This page is about a different car, the 2019 Buick Cascada, 2018 Buick Cascada, and 2017 Buick Cascada. However, it is still accessible from the selected car via links, so may be relevant.
Remove the 4 engine oil cooler bolts (2).
Remove the engine oil cooler (3).
Remove and DISCARD the engine oil cooler seal (1).