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I guess the other question I should have asked is whether it is a 3 or 4 cyl 4000. I will assume it is a later 3 cylinder. If so, then I would bet that the bearings are worn in the pump. When this happens, the gears are no longer able to stay close to the side walls of the pump body. Therefor, when the oil heats up and gets "thinner", it is easier for oil under pressure to escape past the gear face and go back to the inlet side. Remove the pump and clamp it lightly in a vice. Grab the drive gear and try rocking it back and forth. If you see noticeable movement, the bearings are worn. Last I checked, you could not buy new bearings anymore, so that pretty much leaves you to finding a different pump. A heavier weight oil might help in the short term, however, it will not be a permanent solution, the pump performance will continue to deteriorate. Also, a heavier weight oil might not pump so well in cold weather.
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