Ideally you should be using outside combustion air. You should be able to close off the inside vent and use the outside one if it is big enough and will not become obstructed. You could install an interlocked combustion air blower that will only run when the furnace does.

You need 50 cu ft of combustion air per 1000 btu of input. What nozzle do you have in the burner? to figure that there are 140,000btu/gal of #2 fuel oil. If you have a .85 nozzle it is .85x140k=119,000btu input. There should be a tag from the oil company indicating the nozzle size. Figure you combustion air requirements from there. BTW, one grill is not enough. The interlocked combustion air unit is the best suggestion. I much prefer the Tjernlund Enforcer over the Fan in a Can.