I concur with the 3-4 month changes, with the driving that you do.
Oil recommendations will depend on where you live - I live in the south and use either 15W40 in the "winter" or 20W50 in the summer. (the only reason I use 15W40 in the winter, is that is what the 300D uses, so the two share the jugs of oil). The brand (in my case Castrol) is the same for both weights.
The transmission doesn't care too much about how far you drive - it is never exposed to the same stuff as your engine oil is. If you notice that it is slow to engage into reverse, wait for it...don't rev it to force it into reverse, or you won't be happy.

The bigger issue to keep an eye on, are your caps and rotors - they like to be kept fresh - you will change them long before you change wires and plugs.

Enjoy your 124.034!

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D