so what was the problem?

I finally got around to changing the engine oil and filter on my recently acquired 1999 AGCO-Allis 5660, (with only 327 hours on it and an oil/air cooled engine) as I wanted to start using it to move a compost pile and manure stack.  I noticed that every time I started it I heard noticeable valve train clatter... but didn't think about it too much as the "clatter" was gone in 3 or 4 seconds. On Tuesday last I dropped the oil and noticed that it was rather clean to the eye at least and didn't appear to be contaminated, but the filter was a bear to remove..like it had been on the tractor for quite some time. However, once removed the oil filter felt like it had lead in it... somewhat heavy..even after making sure the oil that could drain..did drain out. I replaced the filter with an AGCO original equipment filter and filled the crankcase with JD 15-40 ..Hy-Guard II. Started it up to check for leaks and thought the engine sounded a bit quieter. After a day sitting in the shed I started it up to do some work and quickly observed no initial valve train clatter and a noticeably quieter running engine. Was the oil just worn out and/or was the filter partially plugged up?  I'm starting to believe that maybe no one ever changed the oil and filter...since new.. Or maybe some second rate crankcase oil was used??? thanks       

Thanks Ed, I didn't think that a restricted/bad oil filter would do that .. but it is a reasomable explanation.  The "clatter" was like hydraulic lifters collapsed and needing to be pumped back up. But the 5660 engine doesn't have hydraulic lifters.

I guess it doesn't take too many hours of usage to plug a filter that measures 3 inches long by 4 inches wide. And after I took the filter apart it looked, to this untrained eye, like it was probably plugged up somewhat. The owners manual says to change the oil/filter after the first 50 hours and every 150 hours after that. I suspect that the last time it got changed was in 1999 or 2000 after the first 50 hours. The original owner traded it on a larger tractor after a few months of use. The second owner used it very little the ensuing 12 years...averaging about 20 hours per year.  I guess I should have expected it as it took me almost 2 hours to get a number of loader pins to take grease. I wonder if you can get a larger filter for the 5660... as it is only about 60% the size of the filters I have on my 5000 series JD and AC 175.        

It probably was your oil bypass opening. if you filter was clogged bad