Try dropping the lower oil pan to see the pick up screen. It might be clogged with sludge, or there could be crud in the oil clogging up the screen after it runs for a while.

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82 300SD 202K
82 300SD 233K
83 300TD 340K
82 300TD 98k euro (parts on the hoof)
85 300TD 282K
83 300TD 197K