janos,
every 30000 miles for fluids is the service manual recommendation for the w126's I would recommend 30000 or two years for the fluids (radiator, tranny, diff and brake fluid)
Oil they say every 7500 but I would recommend at least 5000 miles.
m

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Martin Ingram
Colorado Springs
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)