A portable 4 stroke generator, like the Yamaha EF1000iS Generator, for example, I own one since Hurricane Sandy, Amazon link , if you keep it brand new, they are sold dry (without oil or gas) and come with only a light coating of protective grease. Don't add oil or fuel, don't even test start it. The generator will likely still start after you add oil and fuel. These type of generators are already coated in a bit of oil to preserve them for a multi-year shelf life. Good news, the 40 or 50 year old generator will still start if you are sure to keep a couple of quarts of motor oil in the sealed room as well. Bad News, it's really tough to store gasoline for 40 years, so without gas in 40 or 50 years, it could just be a useless fossil fuel antique.