The smoothest diesel I've ever been around is the 5 cyl in our old L3750. Not seen any other 5 cyl engines in tractors. This thing is completely vibration free.
My old L245 (2wd no FEL) needed a couple hundred lbs on the front mowing with a 5' 400 lb bush-hog. Still didn't steer very well, was just too light up front.
I've heard complaints about small 30-something HP tractors with cab, AC, HST Not having enough power. I'd want at least 40hp in a rig like this & personally wouldn't consider one under 50hp...
It could be that the oil pressure is good until the oil gets up to temp then gets thinner, thus the 10 minutes. I'd get a gauge and screw it in where the sender is and read the pressure. If good, replace the sender, if bad, rebuild the lower end.
I agree with taking the loader off. My loader stays off unless needed and the tractor works better in every situation...
Also set your rear tires out widest setting if ya have Ag tires.
I'm kind of in the same spot. I bought a L3800DT because I liked the size and I could pay for it out right, got a great deal also @ 15K.
I love the L3800 & will not get rid of it! But we have 400 acres and I'm mowing more than I had planned. This tractor will pull a 6 foot rotary very well BTW...