"Sludge" is somewhat ill defined and conjures many different pictures. Sludge can be gooey, paste-like, hard, or even gritty. Although most often associated with neglected or poorly-tuned internal combustion engines, it has been known to occur even on modern "clean burning" engines.
https://www.consumerreports.org/cro/cars/car-parts-and-accessories/engine-sludge/index.htm Some sludge might also be an unavoidable byproduct of internal engine detrius, following common engine repairs, including valve jobs or cylinder head replacement. It is also probably fair to say that most older non-computerized engines, if run long enough on gasoline, as opposed to something like LPG, will develop a thick film of deposits in or around the bottom of their oil sumps. Of course, frequent oil changes with decent oil and filters minimizes the problem, but it may not eliminate it. One measure of engine health is the color and smell of its 20W-50 life blood. (However, this is probably not the most accurate determinant. Oil analysis is probably far better, but generally more than one test is required for the most accurate assessment.)
You did not describe the sludge in detail, other than offer a side order of silvery gravy, which is likely babbitt material. This may be insignificant - or not. Flushing the engine has numerous pros and cons and one suspects that it tends to be a choice mostly for those engines with hydraulic lifters suspected of oil flow issues. In any event, prior to reaching for the flush, if you have not already done so, consider removing the valve cover and carefully examining the valve train for sludge. Sludge found on the valve cover's underside may contraindicate the use of flush. Pay particular attention to the condition of the cam lobes. If they show any significant wear, flush may only enhance that wear. (Should something in the oil manage to plug any oil spray bar orifice/s, the unpleasant results are typically observable on the camshaft.) Should you find the sludge to be "gritty," in the pan but not high near valves, it might be prudent to remove the sump pan and physically clean it, before or even after any flushing procedure. Be forewarned, "flush" may loosen rather than dissolve grit that never makes it to the filter. Where it comes to rest and in what condition remains to be determined. Although kerosene may be a simple go-to, and may be a primary ingredient found in most commercial flush products, if I were to use any such product, I'd be more inclined to try a formula that has been around for a while and marketed for that purpose, e.g., Rislone, Wynn's, Marvel Mystery Oil, etc. Having said that, ATF has been known to work on farming equipment.