I finally upgraded to a "Larger Generator" As the 4000watt was feeding all my 110 but was pushing it.
I have in the past run the old 1993 8HP Cast Iron Sleeve Briggs for 7 days with oil changes every 20-25 hours of run time that I bought new.
Since Briggs and Stratton is no longer what they were I have changed over to Honda when I buy any new small engine equipment.

I posted above due tp the fact that when used it will be used in the south and in heat and may end up getting used hard
Plan is to run it as a brake in and swap oil.
The tech says the GX390 13HP takes 10-30 Dino but you can use 5-40 In Dino or Mobil One.And that the most important thing is what I am doing. To run half a tank to full tank of gas as brake in and do the oil change. That most just make sure that it rind and pick it up. He must have had time on his hands as I just wanted to know if they had recomendation on oil grade of other than what it was shipped with.
I did look at Bobistheoil guy and some said they used 10-30 and 5-40 but no one said they used 10-40. So assuming that in the 10-40 the 10 is to thick for tight clearances. And that they saw some gens fail in Florida after Hurracanes with use of the 10-30 in extened run times in hot summer southen heat.

Thoughts