It's hard to mess up the Differential fluid change. Caution, loosen the fill plug first. You don't want to drain the oil and then discover the fill plug is in there too tight.

Based on your question about Air Conditioning, it is clear you know nothing about these procedures. You had better get good step by step instructions before you start loosening valves. Dangerous pressure involved. Not as simple as screwing on a can of refrigerant. You are dealing with a very expensive system and you really don't want to screw it up.
The system is designed for a specific quantity of refrigerant. The only way to get it exactly right is to have a shop that has the equipment suck everything out and replace with a specific quantity or refrigerant and oil.

Steve