Quick question what is the difference between gear oil’s

Differences- manual trans vs rear end. A trans has moving parts, it shifts, heavy oils slow down shifts and the wrong oils degrade brass syncros. Rear ends are usually happy with standard gear oil except if you have certain unusual ones- like a Mopar Sure-grip. Sure-grips have clutches. Too slippery an oil and the clutches won't "bite", too heavy and they won't work either. The lesson is- the right oil for the job. Both rear ends and trannys are usually "full" if the oil level is within 1" of the fill plug.