We had an old Dover like that too. You could clean the piston and it would be fine for a week or so. This car ran really hot and we did end up replacing the oil. When we asked around just about every mechanic that takes duty had added soft stop additive over the years and some more than once. Just a theory but I think that was what was leaving the sticky film on the piston. Like there was way too much in there and it was breaking down from the heat.
