Say all you want, get as technical as you want, list all the "right and wrong ways", and tell me how maybe "not good" it was to do what I did and be sure to show how well versed you are about the subject, but, here is what has worked for me in my 1990 240.
A/C was not cooling well. Low on R12 refrigerant. Got a Freeze 12 kit, which included 3 cans of Freeze 12, a can of a/c oil, the 2 adaptors, the hose and valve set up for the cans for about $50 including shipping. Set it up in about 2 minutes at home. Added the Freeze 12, one can, mixed with the existing R12. Works great, and been working great with no further a/c coolant loss or any problems with it for almost 12 months now, and we use the a/c quite a bit! Maybe I am lucky, who knows? Bottom line, works for me and works good. Had a friend did same, gave him a can of which he added to his system. Bottom line for him, no problems on his either for about 9 months now. Will it last? Will we "be sorry"? Who knows? I'll report back in another 12 months.
BTW, I did call the Freeze 12 company. They said LEGALLY you are not supposed to mix any a/c refigerants, and could not advise anyone to do so, but the person who I spoke with said their company has received numerous reports of success in mixing Freeze 12 with R12 by individuals, and no reports of problems on the mix, but she still said "We emphasize doing that is not advisable and is illegal!" LOL.
Up to you to believe that what she said as fact.
Lot of verbage on this on Ebay.

Running good and running cool!