Without a schematic, troubleshooting is just a WAG especially not knowing ambient temp and what temp the oil actually is. If the oil is 140F I would be somewhat concerned. I would send an oil sample off to a lab to determine both the cleanliness level and the life of the oil. A cheap thermometer can get you close on oil temp. If you can post more info maybe I can be of more help. Good luck,
As given above, I'd agree that oil analysis is needed to check oil condition - typically viscosity, oxidation and TAN will give an indication to how much thickening the oil has suffered. At 4200hrs - this is about 6 months or so on oil - again the oil analysis may indicate if the oil is being over extended.
Also check what oil is being used and if this is the right one to use.