not always a good idea..
I have seen more engines fail after a "flush" than ones that ran better afterwards..its not a good idea to loosen old sludge and tar in a crankcase,and circulate it throughout all the oil passages,bearings,and other parts like hydraulic lifters..the oil filter only can catch so much crud,then the by-pass valve opens and allows UNFILTERED oil to flow all thru the engine..acts like grinding compound and clogs small oil passages..
I've seen lots of old antique cars and tractors,etc ,that were run on old fashioned "non detergent" oil die a rapid death after being changed to a high detergent multi-weight motor oil,especially if they sat idle for a long time before being awoken...
We learned the hard way at the junkyard to pull the oil pan and valve covers and clean out as much of the muck BEFORE starting the engine as possible..then if its still on the dirty side,use non-detergent oil,not newer detergent oil..synthetics usually bring on a quick seizeure!..a few cars we even steam cleaned the INSIDE of the motor and blew it dry with an air hose!..and they ran great after we got them fired up again..I bet the motors would have smoked and seized, had we just got them running and changed the oil,judging by what came out of the oil pan and crankcase..
But my dad had always told how they used to run the older cars on only a quart of oil, and the rest kerosene at idle for 5 minutes ,then drain it and change the oil..I suppose if its done on a regular basis,before a lot of sludge built up,it was a good way to keep it clean..seen it ruin a lot of sludged up motors though....at the least,hydraulic lifters often clatter incessantly after such a "flushing"....and often NEVER quiet down ever again...
Marvel Mystery Oil was also a favorite shade tree additive that cleaned out gunky engines..it has a very high film strength,higher than most heavier oils,so that would help prevent scuffing and wear due to low oil film strength..but dirt would be the biggest cause of damage in my estimation..
Todays oil is much better than our "fathers oil"...it does not let much goo build up like the old stuff did,unless you never change it,or overheated the motor and didn't change the oil..other than that,most of the motors I've had apart the last 20 years have sludge only under the intake(valley pan/lifter gallery) and a bit on the heads under the rockers,and at the bottom of the oil pan..
A little sludge can actually be benificial!--I've found more than a few valve stem seals,timing gear nylon teeth,and hydraulic lifter pushrod seats and their retaining clips trapped in the goo at the bottom of the oil pan..if those peices had got into the oil pump,or wedged between the rods and crank,you can bet there would be carnage!...
