I noticed oily spots on the driveway where I park my 1983 240D. I first thought it was just that the drain plug wasn't tight enough, so I tightened that. Still leaking oil. I eventually jack up the car and discover oil beading up on my transmission. Is this really bad? Is there anything I can do about it?

Depends how much, and where its coming from.

If it's tranny fluid, keep an eye on your dipper level over a few days.
If you need to add more every week or so, you have a problem.
But the seals, especially the front pump seal, are known to slow leak on these. Not a big issue if its slow and you keep an eye on it.

If it's oil, may be a host of things, but check for weeping oil cooler lines. Try degreasing under there to see where its coming from.

Automatic or manual?

it's a manual and it's definitely oil, I don't where it comes from exactly, it just beads up on the transmission.

could it be brake fluid from the clutch/slave cylinder system?

__________________
2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
-------------------------------------
'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold