German rubber isn't nearly the quality that Japanese rubber is, at least for longevity anyway. Japanese 0rings last forever (like the valve cover seal -- ever see a japo car leaking oil around the valve cover? NOT!

German 0-rings are shot every time you take one off, it breaks into bits.

Armour all or any other silicone spray will help considerably, as will vinyl preservers -- they keep some of the UV out of the rubber so it doesn't depolymerize.

Best treatment is to wipe everything rubber down with silicone spray each oil change, including heater and radiator hoses. They last longer that way.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!