I always change the oil and filter at the end of the season right before winter layup. I'll dedicate 3 or so hours to do that, going for a quick run to warm the oil to change out. Afterwards going for another 10 minute ride to circulate the fresh oil throughout the engine. Once on the trailer - but still on the water, I'll disconnect the fuel line and run the engine while fogging until the motor dies from fuel starvation (to avoid fuel going bad and gumming up in the carb). After it's completely out of the water I'll pull the block and manifold drain plugs and drain all the water from the engine while towing for storage and carry on with the rest of the winterization at storage. As for oil I use She'll rotella 15w-40, it's cheap, has good reviews and readily available. I boat 30-50 hours so conventional oil is fine for me. If I boated more hours per season or used the boat in very cold temperatures then I'd consider synthetic..