For the most part in my experience it liquifies deposits. If you are going to clean a nasty engine with an additive you would definitely want to shorten oil change intervals I would go as far as to change filters between the shortened intervals.
My dad had a 77 Ford Granada. I changed the valve cover gaskets for him once. The valve covers will full of sludge. It wasn't over heated and it didn't have coolant in the oil. Some egines are more prone to sludge, but oils have improved a lot since the late 70's.