Now you have a different story. If you are going to use them some more, then yes, change the oil. True what f100turbo says, oil does have acids. But the genny is open air system, (no pvc) and new oil will get full of moisture in storage, and need changing again in the spring. Motor oil recommended change interval is 6 MONTHS, or miles/hours,, whatever, ....... The time is due to moisture content. Don't believe me, that's ok, but some new motor oil, pour in to a glass jar, take a picture, store in you basement for two months, and take another picture, notice the color difference. That would be water.
There will be other opinions, I have mine, they have theirs.
Certified ASE Master Technician
2012 Ford F250 ext. Cab 4x4
2002 Jeep Wrangler Sahara
2008 Yamaha V-Star 650 Classic
2008 Work and Play 18LT