My I and T FO-41 covers the 10 series which is my 3910 and probably your 4610. Having wet brakes and clutch pack Independent PTO are 2 of the current tractor amenities requiring Premium oil which puts you in the 134 C,D range which also shows up as JD J20C on the applications label. So I use Premium in my 3910 whereas the other Fords use regular UTF which is the 134 A,B spec.
On your tractor sitting, next time you change engine oil, with the oil out, run your finger into the oil drain plug and feel the floor of the oil pan. You may find a layer of sludge which keeps your freshly changed oil black. I dumped a 16 oz can of Sea Foam (any auto parts store) in my 3910 when I bought it, having only 900 hrs over it's 30 years, making for a lot of sitting around and a lot of contaminants settling out of used crankcase oil winding up on the floor of the oil pan. I ran it for about half an hour hard and while still hot drained the oil....felt the floor again and it was clean. Put in new oil and filter and oil stays clean now.