When it comes to cruising, a man’s best friend isn’t his horse, it’s his engine. Diesel auxiliaries build up acids in old sump oil. Leave the stuff in there all winter and these munch away at its vitals, so the best policy is to change the oil in autumn. While you’re at it, change the filter as well. Most modern units make these jobs easy for an owner to manage, and most yards have oil disposal facilities. It’ll be one less task at fitting out time too.