You can and even should use engine flushes if you or the previous owner overdone the oil-change interval by at least double, or if you would re-oil your engine from mineral-oil to synthetic oils for instance
(unlikely these days, as basically everyone uses synthetic or semi-synthetic oils - even in close-minded germany).
This at least in Germany is a standard procedure in every single shop i know and the results are quite positive (first hand report of an E420 with a 30.000km overdue Oil-change available).
Even further, from two Toyota Dealers/Shops i know that they always do a Engine flush with "Eurosol Öl-System", on every single oil-change of their customers.
The M119s due to the way they are driven (in europe/germany at least - full throttle) are prone to Oil-buildup in the engine head and oil-pan area. Don't know what proper english word there is available for, but in germany we say "Das Öl verlackt", "The Oil transfoms into lacquer" ???
Anyway quite some M119 driving People in German Forums observed and reported often, that even with the stock 10.000km Oil-Change interval done, but with a mineral-crap 10W-40, the engine gets this "Oil-Lacquer" builtup quite fast in the head area, covering in later stage even the Head-Bolts and making them practically unaccessible.
Bernard showed once in the German Forums what happens if you overdo your oil-change big time on the M119 (IIRC it was overdone by at least 50.000km), The engine was so full with "Oil-Lacquer" that he couldn't even get the heads of the block anymore, as the Head-Bolts were covered in centimeter-thick "Oil-Lacquer".
I try to get the pictures and link them here 
EDIT Pictures from Bernards W140 Forum:


So for a well maintained and clean-looking M119 i would NOT use an engine-flush. But if you are from Europe, fresh bought your M119 Benz and the Oil is looking already pretty dark even black, do a oil-change and use such a engine-flush.