My plan for long life for my 115 HP Mercury is a gas to oil ratio of 48 to 1 (one pint of Mercury Premium Plus oil for 5.75 gallons of BP 93 octane), use of Sea Foam in the gas tanks and Mercury Power Tune in the combustion chambers and carbs each fall to get rid of deposits, change lower unit grease in the fall, storage seal, clean sparks plugs every year and change every 3rd year, grease fittings yearly, accelerate gently from the start rather than give it full throttle, limit RPM's to 5,300 or 5,400, wax the motor frequently, and degrease the powerhead.
Change the water impeller, clean the distributor and contacts and spark plug connections as needed.
I maintain a log of the maintenance work since the motor was purchased in 1987 which helps me know when to change what, part numbers, and where I purchased the item.
This might be overkill, but it works for me, and yes, I am an accountant.