My manual suggested the first oil change at 20 hours. and change every 100 miles. Page 43... Suggest you double check your owners manual and reread the intervals.
If you are camping in summer... I'd be inclined to go with a 5w-40 full synthetic, something like Mobil 1 Turbo Diesel Truck. It's a motor oil that will go 100 hours, with good additives that handle the heat an air cooled motor can generate. It is both gas and diesel rated motor oil.
I generally start my motor up, do a battery charging with it, so it's good and hot, then turn the motor off, flip the vent on the gas cap, open the cover up with a quarter coin, and undo the top, pull the cover off, open the oil cap, and tilt the generator over on it's side to empty all the oil out while it's good and homogenized and hot, into an oil recycle container, for a trip to Pep Boys to recycle it. 400 ml sounds a bit shy, I thought the motor holds more oil than that? Closer to 500 ml? I usually fill it right up to the edge on level ground. Put the dipstick in.
Honda EU2000i Owners Manual and service recommendations.Edit, yes, it is 400 ml of oil. Like others have stated, with no oil filter, extended oil changes on an air cooled motor is not a good idea. Stick the the factory intervals. Honda encourages more frequent oil changes in dusty, dirty environments, which included sitting on dirt in the forest, or desert, where the winds lift it and get in in your air filter and oil. Elevate your generator off the ground by 3 feet if you want to keep your oil and filters subjected to a less harsh environment.