How can coolant leak into your 1996 BMW 328i oil?

Most likely, your head gasket no longer seals properly. Coolant and oil normally circulate through separate channels in your engine. A faulty head gasket allows oil to pass into the coolant channels and vice versa. If your gasket is leaking, you will also have loss of compression in one or more cylinders and possibly white clouds of steam in your exhaust, from coolant getting into the cylinders.