According to the TSB (MBNA 18/11a, June/1991), the O-rings prevent the oil in the filter housing from going back into the pan after the engine is off. The subject of this TSB is "Engines 602, 603 inspection of oil filter O-rings". It instructs MB tech to check the rings first if the 602 or 603 engine has noisy hydraulic valve tappets "during and after starting (especially with a hot engine)" before replacing the tappets. If the ring(s) is damaged or hardened, it should be replaced. Picture (60x engine) shows both rings are the same size, P/n 015-997-94-48.
[This message has been edited by be459 (edited 09-19-2000).]