In the 60's, when you could use a 160 thermostat and combine that with a pariffin based oil and around town driving, you could almost guarantee that the valve train would have a thick layer of creamy crud build-up. Condensation build-up in the crankcase, never getting hot enough to burn off the moisture, the auto-choke dumping in fuel past the rings, which diluted the oil and the pariffin oil, turned to creamy butter! It didn't take much in-town driving to generate a real mess. Automatic transmission cars were worse than sticks.
Today's better oil blends, 195+ thermostats, EGR systems, and the accompanying higher operating temperatures has done much to eliminate this condition. Most high mileage modern engines, even with extended oil change schedules, rarely accumulate the type of crud we found in our older engines.
Oil and filter changes in a decent running 235, along with a 180 thermostat, at 2k should keep the ole girl running. Limited mileage, around town would have me changing both at 1K or twice a calender year. Oil and filters are cheap insurance! I also like Shell Rotella with a little higher zinc. Hope this helps answer your question.
