Onan oil change now not running

The correct oil pressure is zero. It's a splash system.

Another possibility is oil pressure. Asked for dino W30 and W5-30 full syn was used. Can't build up enough pressure.

How about the possibility not enough oil has been put back in and the low oil shutdown is keeping it from starting?

So it's a 4KY? What is the Spec, the Letter at the end of Model Number? They're a splash-lubed motor like a mower, and the Oil Sensor was deleted before ours, made in 2002 and Spec "K" so odds are the oil change didn't do it. Unless maybe it's overfull and oil got where it shouldn't be.
Did you have the Carb off? Pretty easy to goof the linkage up.
Anyhow, please post the full Model (with Spec) and Serial. If we're lucky enough for a generator tech to drop by, s/he will want it.

Yup. Overfill is a bad act.

Is the oil overfilled? I've seen low shut down engines also die because of too high an oil level.

And changed the spark plug during oil change. After wouldn’t run even swapped back in the old plug. No go still.