The oil capacity for a 1977 GMC 6500 with a V8 engine isn't a single number. It depends on the *specific* V8 engine installed. GMC 6500s in 1977 could have had several different V8 options (like a 350, 400, or 454 cubic inch engine, for example). Each engine had a different oil capacity.
To find the correct oil capacity, you must locate the engine's identification number (usually found on a plate on the engine itself) and then consult a repair manual specific to that engine and year of the truck. The owner's manual (if you have it) would also list this information.
