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.
sidenote:
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
)