Valve yes.. Oil ring. I wouldnt think so. if your valve has something wrong with it, you are not getting the most compression possible, not to mention the gasses would be blown back out the intake or out the exhaust when that piston is on the up-stroke. you should have been able to tell something by the look of the spark plug possibly. but since they fiddle fuddled with them it's hard to say without driving it for awhile and then pulling a known good plug from another cyl and the number 8 and comparing the two.

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed