I have a Focus 1.6D 2013 manual car. Heard someone saying this engine is a carbon slushy. I am regularly changing the engine oil at 4000 intervals. For the next one, I am thinking of giving the engine flush a try. Can't Contemplate the good ones. Your input is kindly appreciated. 

Here's my input. I have owned numerous cars in over 40 years, I have never used an engine flush in my life, and have never had any major engine issues. 

Yours will be the later 1.6 TDCI engine with only 8 valves.  This does not suffer with the same carbon build up as the older 16 valve 1.6 TDCI did.  There's no need to change the oil more than once a year/12k miles on these.

As for engine flush, it shouldn't cause any damage on this engine, but also shouldn't be needed.

Some years ago I used an engine flush on my old Peugeot 405 and refilled with decent oil, after the flush the engine was more noisier and rattled more, changed the oil again after a month had passed, noise and rattles was less apparent, was talking to a local mechanic and he said such products should be banned as it strips away everything within the engine.

Thank you for all the insights, @StephenFord, @TomsFocus& @Ian Lanc. I will refrain from using an engine flush. All those videos on YouTube motivated me in trying it out.. 

In my motoring life of 40 years I have only flushed when the oil has been contaminated with coolant when head gasket replacement is needed. Some say flushing will loosen all the crud so it will block oilways on some susceptible engines.