In most cases generators are fitted with OHV 4- stroke engines that require a good quality 10w40 or 20w50 such as Yamalube 4. In some areas of N.America where extreme cold tempertures are found, it may be necessary to use a 5w30 or even a 0w30 Yamalube product to ensure easy start up and provide the necessary engine protection during start up and periods of extended run time.
When changing the oil, run the unit to warm the oil and then remove the drain bolt,then refill the crankcase with the desired grade of Yamalube 4. Be sure to recheck oil level on a flat surface and remember that Yamaha generators have a low oil switch, were they will not make spark and run until the correct oil level has been achieved.
I know this is an old post, but just in case you haven't found a good solution for a mess free oil change, here's what I did. It applies to most models. Just need to check your drain plug size and thread pitch.