If you can see sludge build up, then it's already too late even to use regular off-the- shelf engine flush. Or you may have to do many times before you can see any improvement.

Yes, that's why I do it at rather regular intervals even though not too often like oil change. I've done that for all my cars and even those which were >10yrs old, the internals were still shinny gold and no sludge what so ever until I sold them. If those which use hydraulic lash adjusters like Mitsubishi engines, then maintaining engine cleanliness is even more important to keep those HLAs live longer and still sounding smooth/soft. I learned this the hard way during my first ownership of a Lancer back in the mid 90s.