The rear seal holder for the 1995 and up Buick 3800 VIN K also functions as the cover for the coolant passages and oil galleries on the back of the block. This can cause problems, because the coolant passages are pretty close to the oil passages, and the original covers (c/n 24503969) tend to corrode around the coolant holes. Once the gasket gets saturated with coolant and the cover ends up corroded around the coolant passages, you can end up with either water in the oil or oil in the water.
Inspect these covers carefully and discard them if they are badly corroded. Replacing the early cover with the later, revised version (c/n 24507385) that has a more distinct ridge around the coolant holes to provide a better sealing surface for the gasket is cheap insurance. This updated cover is available from GM (p/n 24507386).