had many 350 car and truck gasoline engines over the years...
it was always 5 quarts w/ filter change.
I'm not aware of the 350 bus engine being any different than one of them, except 2 Bolt Main and 4 Bolt Mains.
but, I'm no where near an expert on bus engines.
If it were me, I'd change filter and start reading the dipstick after putting in the 4th quart.
Now, my 2005 suburban's oil capacity for the 5.3 liter is 6 quarts w/filter change... but it's the engine where they mounted the oil pump behind timing cover (non-issue for 1988