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601, 602, and 603 engines have "bucket" tappets (also known as "valve clearance compensation elements") that wear out and "wobble" causing them to stick, thereby setting up a condition where the hydraulic lifter "ticks" on the tappet. Stouter oil sometimes greatly reduces the annular space due to wear around the tappet, quieting down the "ticking." Of course, once the oil thins out from mileage, the ticking starts back up. Not totally related to the OP, but useful info for folks w/later engines where this might be occurring.
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