a couple of Champion questions,transmission etc

I have a 1991 Champion 740A with around 15000 hrs,trans replaced with a "takeout" about 1000 hrs ago. Ive had it 4 years and maybe 300 hrs. Trans whines in 4th gear . Is this normal? I dont like it so I shift to 5th quite soon. (Its the 8 spd power shifting trans.)No metal in the trans filter. Doesnt seem to be getting worse. Is this normal?
2nd question. It seems theres a seal leaking in the diff. Diff oil goes down ,LH chaincase goes up. I have to add a gallon or so when I go to grade every 3 months or so. It seems to go down to maybe 2.5 inches below full level I think thats about the level of the bottom of the axle seal on diff.It was at least that low when I bought grader so I blv prev owner just let it stay that low. Its the big diff with lock. Holds a fair amount of oil even when low it appears. So Im thinking do i need to keep topping it up OR can I just let the chaincase get plenty of oil in it and the two will kind of level out. Cannot really see what a bit(lot) of extra oil in chaincase is going to hurt.
Opinions please?

I don’t think that it’s a real big job to change out that seal between the chaincase and the differential case. I wasn’t there when the mechanic did it, but we had one done on a series iv and he was out of there in good time. Does that one have drum brakes or disc? I’d be concerned about leaving it if disc brakes. The seal for the disc brake piston can swell and drag.
On the howl in certain gears, I’m not sure that’s normal, but it may not be atypical either. I think some of them howled in different gears. Those early ones were apart often before they changed up to a more aggressive clutch material. Once they figured that out, those transmissions ended up being bulletproof.

What would "early" be for one of these units be I wonder?
I have the drum brakes so no worries there I guess.

Well, I ran a 1990 730 that had the transmission rebuilt several times. The material on the clutches wouldn’t last. I think it got rebuilt the last time in 1995 with upgraded frictions and it never had to come apart after that. I ran a 1994 740 up to the 18000 hour mark after that and that transmission was never touched. So somewhere’s in between was when they made the change.