The amount of oil a 1968 Chevy engine holds depends entirely on the *specific* engine. Chevy offered many different engines in 1968, from small inline-sixes to big-block V8s. Each engine has a different oil capacity.
You must consult your owner's manual or a repair manual specific to your 1968 Chevy's engine to determine the correct oil capacity. The information sticker under the hood might also provide this information.
