I remember feeling really concerned about that the first time I did my own valve adjustment. 6000 isn't that long... I'd reuse it. If it weeps, throw on the new one. Doesn't take that long and you might end up saving some rubber for another day.

Re: gaskets, same deal. I've found the Victor Reinz gaskets to be so/so... meaning, I've had some leak and require adjustment and or RTV (not my job), others have had no issues. The Mercedes gaskets, however, have never given me any issues.

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1981 240D 4sp manual. Ivory White.