Flushes for oil remove any sludge from the part of the engine where the oil flows through, there is unlikely to be much built up sludge unless you go a long time between oil/filter changes. Sometimes sludged up oil is sealing an old engine and it may begin to leak afterwards but this is a symptom of a bigger problem - so if you flush it out be prepared to check for leaks in the weeks afterwards and replace gaskets/seals anywhere you discover oil coming out. Check the exhaust for darker smoke that would indicate oil being burned, which would mean you have to change the piston rings and check cylinder compression, although I would say this is extremely unlikely that your pistons are sealed by dirty oil.
The other kind of flushes (the kind that you spray below the air intake/throttle body) are supposed to remove carbon deposits from inside the cylinders and exhaust. You might have heard of seafoam for small engines like lawnmowers although I think this is an american product. There are mobile services advertised where somebody comes to your house in a van and does this for you.
There is also coolant flush for the engine coolant, this removes deposits from the holes within the engine casings where the coolant flows through, as well as the water pump, the radiator, the hoses and the heater. You buy it in a little bottle, remove some coolant from the radiator (using a turkey baster or similar) and then add the whole bottle. I made the mistake of adding it to the expansion tank once which doesn't work, it just sits there.