Engine oil seals are made of hard plastic resins with added plasticizers (usually diesters) to make them flexible. These diesters are very similar (in some cases the same) as those used as cosolvents in older synthetic engine oil formulations. Since these plasticizers are oil soluble, they will leach out over time. This causes the seal to harden, shrink and form gaps.
Modern high-mileage oils have a pinch of diester that helps replace that which has leached out to keep the seals flexible.
I have a '90 Mazda Miata I bought used at about ~140,000 miles that tends to only come out on good convertible days. Early on, it would smoke a little on start-up if it had been sitting more than a couple of weeks. I switched to a high-mileage oil and now I don't observe any smoke, regardless of how long it sat.
Unless you have solid evidence of deposit problems, I'd avoid engine flushes and just go with a good-quality high-mileage oil.