Compared to a modern engine, that one's going to have oil clearances you could throw your hat through, most likely. I'd stick with a good quality 30 weight detergent oil, and change it often enough to keep any sludge from forming since it probably doesn't have an oil filter. a 6-month interval on oil changes, regardless of mileage will prevent any condensation or other nasty stuff from accumulating. Also be sure to run it for a long enough time on a regular basis to come to full operating temperature. Short runs that don get the coolant hot tend to form lots of condensation and sludge.
Jerry
