I’d say there’s none there, based on the images and also the oil spreading from the plug out. You should be able to see washers from the side if you look at that angle, and I don’t see any.
The title you chose suggests that it was only loose, but there was a washer missing. Was it comfortable when you tried to tighten it?
If it hasn’t leaked before, it’s their responsibility to fix the problem, not yours. I would show them the images you showed us.
You won’t be able to put in a new washer without major oil loss, BTW, which would be like trying to change a washer faucet with the water not turned off. It will need to be drained and refilled.
