Personally, I would change the oil more often, say 50 hours. There is only a bit of oil in the crankcase...less than a litre so it takes a beating, especially if it is hot.
I would get a hand vacuum pump, then simply remove the side cover, stick the pump pick up in the dipstick hole and suck the oil out. Then refill, but don't over fill. Very easy and clean. If you don't wish to beg, borrow, buy or steal a pump, pull the drain plug and let it drain into a suitable pan.
This is the one I like, but there are lots out there even cheaper. I use mine to do all my oil change stuff, cars, boats, stationary engines etc. Best tool I own nearly.
http://www.amazon.com/Mityvac-7400-...&qid=1440618066&sr=8-4&keywords=Oil+extractor
PS Warm the oil by running the genny for a bit before you change.
Icarus