Proper job-fixed/re gassed 1000’s of systems on my career and you can not do a good job with one of those top up things – you need to vacuum the system for it to draw the new refrigerant in, Plus as mentioned it will highlight any leaks or compressor issues. The correct way is Recover. vacuum, hold (Checking for leaks), recharge. Whole process takes about 45 minutes depending on where the Hi/Lo system connectors are hidden :-) You only need to inject oil if the compressor or another significant component has been changed. Dye can be injected during (as part of the automated process) or separately afterwards.