!985 300CTD;150,000miles. During a 7 hr trip at highway speed the oil pressure indicator needle stays at a constant "3". When reducing speed & coming to a stop or idling the needle bounces (is not steady) at just under "2" & a "thumping/knocking" sound comes from the gauge area but is not heard when the engine rev's up. The engine runs quite well & pulls strong. The oil/filter was just changed & I have used synthetic diesel 15w-40 in the vehicle for the past two oil changes@5000 miles per.