Just a question. Its been awhile since I have been around a 45. Is the priority valve only on Number one valve? On our MT A series Challengers we can click each valve to be priority or not and the newer ones I believe are all priority. When you put the fan on number 4 and the down pressure on number one then it is going to supply oil to number one first. As others have said turn down the flow on you down pressure and see if that makes any difference. Was it run long in the first place to get the oil up to operating temperature? In some weather conditions our MT Challenger surges the fan until the oil gets all warmed up to operating temperature. Anyway throwing out a couple ideas.