There is an old saying, “Pay me now or pay me later”. This is especially true when laying up an engine for storage. Skipping steps at lay-up will guarantee expensive repairs at start-up.
General: Each manufacturer has recommendations for oil changes, filter changes, gear case lubrication, etc. Find the manufacturers’ recommendations for your engine and follow them. Change any oils (lubricants) before storage. Any moisture in the old lubricant can cause component corrosion and pitting on bearings, which should be avoided at all costs.
Petroleum based fuels are complex compounds with many additives during the refining process – more so now as fuels are reformulated to meet government mandates to lower pollution rates. Unfortunately these same agents that make for cleaner burn make the fuel less stable. Diesel is now less than a 90-day fuel and gasoline is a 30-day fuel. After that the fuel deteriorates and loses a good portion of its efficacy. As fuel ages, asphaltenes form, darken the fuel and eventually settle to form sludge at the bottom of the tank. A good stabilizer slows down this darkening and sludge formation. Old fuel often also contains water contamination.