Oil and some rubber just don't mix. This is what happens the vacuum pump fails on a OG9-3.
The function of the pump is to give power brake assist on the automatic transmission cars. Manuals do not have the pump. The pump is driven by the engines exhaust camshaft. A diaphragm inside the pump fails and allows the engine oil inside the cylinder head to be pumped in to the brake booster. The brake booster gets to the point where it either overflows into the engine or leaks into the master cylinder. If caught early enough, only the vacuum pump needs to be replaced (to the tune of about $275), if left be for too long, the master cylinder and brake booster need to be replaced as well, as well as the vacuum line that connects the whole works ($450 for the booster, $600 for the master, $45 for the line, and about 3 hours of labor @65/hr).
Perfect example of how an issue that starts small can really pile up and cost big money. If something feels off, get your car looked at!
New master cylinder in the background, old in the foreground. See the swollen seal? That's the effect oil has on the rubber seals of the master.