when my 40 still had the 228 in it, it didnt have a filter. i did some work to the top and bottom of the engine(top-cleaning, bottom-rod bearings)and did not add a filter. with regular oil changes the oil was clean even after 5000 miles(two summers). most of the dirt comes from the vented valve cover and blow by. since putting in the 56-270 ive corrected both issues and addressed full flow filtering. before that i just used shell/rotella sae 30wt oil. when i didnt like the color of the oil on the dip stick i replaced it regardless of miles which was usually between 1500-2000 miles.
