If I paid a grand for a fairly low output genset, trying to use cheap oil in it would be the least of my concerns.
Since this thing is always warm-stored in your foyer and if you ever need it as an emergency backup you'll be running it long and hard, I'd spring for M1 HM 10W-30 which at least has HTHSv complying with the old A3/B3 spec.
I don't think I'd want to use M1 5W-30 or a Peak conventional HDEO in it much less a mix of the two although the Peak would at least have a high HTHSv.
Heck, the Peak all on its own would likely be preferable to a mix of it and M1 5W-30.
Unlike the engine of a car, the engine of a portable generator will be running near its power peak for most of its hours of use so oil selection is probably of more importance.