Used oil sitting in an engine is not good. It is better to have fresh oil in it. Also, as mentioned the raw water side of the cooling system should be winterized BEFORE it freezes. That is very simple, just put the raw water pickup hose in a bucket of non-toxic -50F antifreeze and start the engine. Run about 3 gallons of antifreeze through the system then shut the engine down. Drain any fluid in the raw wate strainer and reconnect the hose. I would also ctcle all the seacocks and leave them open for a day or two to let any water drain out. After that you can either leave them open or close them. As mentioned above, test the antifreeze to see if it is still OK. You can leave the fuel filters alone for the winter, but change them in the spring