Well, if it is an old engine
running conventional motor
oil, then yes there will be sludge (coke, tar, whatever you prefer to call it) on the valve cover and other places. That's one argument in favor of synthetics--they just don't tar up the same way.
But if you've had no problem from the tar (sludge) and aren't worried about it, there's no reason to run a "cleaner" in the engine
I know some old mechanics used to add a quart or more of kerosene, run an engine (not under load, just static) to circulate that as a cleaner, then drain and change the oil. And MMO has to be gentler than that. It has many fans but I've never heard any loud complaints that "MMO ruined my engine" although I'm sure a web search could find at least one.(G)
an engine just to support a newly installed oil gauge? Seems pointless. If there's a real
sludge problem, in theory a clump can break off, clog an oil passage
, and cause an engine failure. If that thought keeps you up at night, well, time to reconsider synthetics after the cleaning
. If not, no problem.