Ya, the sight glass is just under the operator platform behind the clutch. Invisible Super UDT2 is a poor match for a sight glass, especially if the level is a hair above or below the sight glass.
Is it a HST? If it is, I would only use Kubota Super UDT or Super UDT2. 2 is better in colder climates than normal UDT, but probably not any better where its warmer.
You can be a lot more lax if its a gear transmission.