Just only 1 time late for service won't cause sludge problem.

How late was the service and the oil mileage use that time?

What car? If it's a naturally aspirated engine, it should be fine..

If it's turbo, then that's not a good practice.

My advice is don't flush. If you flush, the gunk might get trapped in the oil pick-up tube which will disrupt the oil flow to the entire engine.

Only do a flush when you plan to remove and clean the oil pan and oil pick-up tube. Else, just do the normally oil change will do.

For me, I would change the engine oil on that Sperma every 5-6000 km and run fully synthetic in it since fully synthetic oil are quite affordable nowadays. No need to go for Motul or other fancy oil.