Every time fuel prices have gone crazy, a question like this rises on the floor at the authorized local dealer service dept.

As far as I understand, converting a gasoline W124 into diesel is an expensive and tortuous job, although it can be achieved with patience, lots of money and tons of expert knowledge.

For the W140, as far as I understand, is almost impossible. And I say almost because "nothing is impossible", but if you do want the conversion to work you basically have to be a miracle maker and own a couple of banks at the Bahamas.

A local independent did switch a 3.5L turbodiesel for a newer 3.0L (for obvious reasons -> the rod bender!) on a SWB W140. Although both engines were turbodiesel, the amount of times the car has had to return for adjustments, calibrations, failed electrics, et.al is as pathetic as the number of times a brand new Land Rover Discovery 3 has to visit the service shop.

I guess this would give you a clue.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]