That rear cap is sealed between the balance shaft cover and block by an o-ring. That o-ring shrinks with age and causes the cap to get loose like that. To change that o-ring you need to take the cover off. Not a bad job for the upper, but the lower is a pain in the butt. You need to take the passenger side engine leg off and drop the crossmember.
Depending how bad yours is leaking, many people have luck with just cleaning the area very well and putting RTV silicone over it and letting it dry a day or so.
I think more than likely every 951 has that lower one leaking due to how close it runs to the crossover pipe. I know mine does..
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