Just in general.
A/C systems have a design oil and r134a charge.
Too little or too much r134a will cause poor performance and too much can cause a hose/compressor failure.
Too much oil will degrade performance. Adding oil to a system without knowing how much is in there is usually pointless. The only way to know how much is in there is to completely evacuate the system and add a fresh, known, charge (This is true for 134a also)
Leak sealer and o-ring conditioner, while they help reduce leaks, they also reduce performance. If the leak stops, it does make it better than no a/c, but it will never perform like new.
I don't know what maxi-cool is, but, if there was an additive that increased performance on this type of system, believe me, my company (Carrier Air Conditioning) would be using it. To my knowledge (A Senior Engineer with Carrier) there is no such additive.
Having said all that, these kits are for really poor performing systems, are are a bandaid to give some performance, not "Like New" performance, regardless of what the marketing claims on the can say.
Evacuating the system, leak testing, carefully weighimg a new charge of oil and 134a is what makes these repairs so expensive. -Having said THAT, many places don't do it right but still charge the high rate -
Finally, many people have winged it, charged their system successfully and are happy with the results.
FWIW, some car fires are caused by overcharged systems that relief the excess pressure through the safety relief valve. The oil, sprayed under pressure, gets ignited by the vaporized oil. So those that told you they did it and had good results, had good results. But it could be different too. BE CAREFUL. KNOW WHAT YOU ARE DOING!
