Valve guide seals commonly go out. They're the cheapest of the my-engine's-burning-oil fixes, and common for them to go out. A google search will come up with a lot of good ways to change them. My favorite method includes a rented spring compressor from auto-zone, and pulling spark plugs & stuffing them with small rope (this will make sense to you after you find pics of the method). It's my understanding the 8v is easy, and the 16v is a PITA.