I changed all the cars to mobile 1, including the ancient 220D in spite of the oil consumption (I won't mention how much!) for the following reasons:

Vastly better starting -- it can get cold in the winter here in S. Indiana and the 220 needs all the help it can get (including the tank heater).

Better mileage -- at least a few percent.

Much better turbo response -- my brother just put Mobil 1 in his 87 300SDL, with the result that the turbo comes on at about 1000 rpm now, and spins somewhat faster. It is now audible in the car and goes supersonic under load like my Volvo (which sounds like a police siren). Also, synthetics will not coke in the turbo and should give much longer life.

Seems to have freed up the rings in the 220 and upped the compression some (I've not tested it)

Drawbacks are higher cost if you still change it too often (7500 miles between changes is fine, you cannot wear it out), more leaks if you have any as it is "thinner" than petroleum based oil at low temp (thicker at high temp!), and sometimes more oil consumption due to lower viscosity in worn engines. Less smoke when it burns, though..... They will clean the engine up, so expect to change filters every oil change for a while to cope with the crud.

The problem with shrinking rubber seals one hears about was solved 25 or more years ago.

I asked a lubrication engineer about synthetic oils a couple years ago while working on a additive project for him, and his comment was that synthetics are simply vastly better lubricants by any measure.

Don't put lubricating oils in the fuel to "lube" the injection pump -- use a good diesel fuel conditioner like RedLine or Diesel Purge, or Shell's, or Chevron's. Motor oils or ATF usually contain things like zinc stearate, which turns into zinc oxide when burned, and zinc oxide is a nice polishing agent -- will polish the honing out of a cylinder bore, for instance. The cam in the IP is lubricated with engine oil anyway.

Diesel fuels vary much more than gasoline, and some engines are more sensitive than others. Buy whatever runs the best, and add fuel conditioner, especially if you are driving a Rabbit or Volvo Diesel -- the VE type pumps are very sensitive to low sulfur fuel (as I found out......)

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!