The Cummins/Onan web site has all the manuals, operating manuals, installation manuals, etc. You will find the information in there.
You are confusing the issue saying in the title it is a 2800i which is the newest inverter model generator, then you give the series as a KV. The inverter model 2800i is a HGLAA, so I have to assume you have a regular old Onan 2800 model. The last character of your model number is your spec letter (also shown separately on the data tag) which in your case is a G.
Operators manual, Engine oil recommendations are found on page 9 (PDF page 11) I think it only holds a quart, but I would buy two to be safe. I use Amsoil 10W-30 small engine oil. Cummins does not recommend synthetic oil but do not forbid it either. They just want you to wear out the engine faster.
981-0153
Installation manual, page 2-2 (PDF page 14) has the spark plug torque, gap and valve lash settings.
981-0604
Parts manual, OK, well Cummins wants you to LOG IN to access this, I hate corporations that want to control this kind of data, essential to owing the unit.
981-0238
I have a 2500LP model spec L, so virtually the same unit. So am I to understand that you cannot even open the front panel of the generator? How do you check or add oil? The air filter, spark plug and the oil fill are accessed thru this panel. The oil drain is thru the bottom, is the bottom exposed?
Word to the wise, do not fill the oil to the very top, leave it about half way up hash marked section of the dipstick.
Second word to the wise, do not use a Champion spark plug in the generator, period. How many hours do you have on this generator? I have 200 on mine and the plug looks like new.
The front of your generator should look like the one in the line drawing (first attachment)
I will do some digging and find the part number of the air filter in mine, which will be the same as yours.
Found this chart on the Cummins Onan website that lists by models, the oil quantity, and part numbers for every thing you need. Yours is the second line down.
KV A-M - 1 quart oil, Air Filter 140-2105; Fuel Filter 149-2137; Spark Plug 167-0263-02; NO Service Kit numbers. There are footnotes, so look at the link to see the whole chart.
ONAN online parts ordering
Charles