I've just started having the same trouble lately. Stored fuel & tractors not running right. Treated all fuel and just bought new filters today. This is the first time in over 35 years I've had to do this...What's up with the new fuel?
First, oil can't be "too clean". :)
Why did you change the oil at only 8 hours?
It takes a while running before the oil shows any color. Run it and don't worry.
Yep, changing to 80/90 gear oil might stop the leak & is what I run in mine.
Seal not to difficult to replace, I'd just do it.
Loader work with no, or not enough counter weight on the 3 pt can play hell on the front axle components.
Usually if there are problems they show in the first few hours. After fixing, the rig is good for many years. You wont find higher quality than Kubota, but good luck with whatever you decide to get...
I have 5 & 6 foot mowers and the L3800DT handles both just fine. What I don't like about the 6' is it hangs out past the rear tires and will hit trees, fence post, etc... Very nice open spaces though.