to check gases it should go to a shop they can do the lambda to although you can do the lambda part yourself search duty cylce there are a lot of good threads.
fluids at least every 3 years execpt oil. This should be done at every 3-5000 miles or six months.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)