I've heard many stories of the W123 engines loosing all of their oil while driving due to the following reasons.

Oil Cooler Rad blows/cracks
Oil Cooler hose is bad

My cousin lost his beautiful 240D due to this, didn't have the money to buy an engine and put it in so the car went to the junkers. So sad for such a small issue of not maintaining the car. m*ercedessource on youtube has a great video on replacing these. I would recommend only buying this part from the dealership. This is quite a cheap replacement from the dealership as per the picture below.

The oil cooler rad is about $700 ordered from Germany, I would recommend having a backup or removing yours and oil spraying it/painting it.

Save the W123's, every year there are less and less on the road

I'm curious, was there any new leakage before the failure, or was it completely out of nowhere?

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

It happens out of no where, I had it happen in my 1976 W115 240D. Hose was fine but the oil rad blew, I was able to pull over in time.

Well....Keeping an eye open doesn't hurt, and if you don't...don't be surprised!!...Problems even such as this usually show a sign of things to come. Many people just jump in, turn the key, and off they go! I check everything I can check...and frequently.