GMC A/C repair..HELP

My a/c locked this past weekend. I took it in for a second opeion, after "my" garage gave waht I thought was a high quote (after all, they take care of all of the repairs). I went to my friend's brother's garage. He looked it over and came back with this report:
"System has about 88.3% R134 and about 11.7% R12. Checked twice to confirm that system had been mixed. Truck has R12 fittings and no decal to show R134 had been put in. A/C compressor locked up."
I am told by my garage that this is normal, as they recharged it 4 months earlier. Is there a problem with that statement? or what? What is going on?
1980 GMC Sub 1500

You are supposed to convert the system first before putting R134 in, that much I know. I just converted my old 91 Corsica and instructions (ALL) I have ran across state that the old R12 has to removed, then you are to put adapters on the high and low pressure side of system. THEN you can put R134 in. In fact, if you do it by the book, you are supposed to replace O rings and oil in compressor. However, many people tell me that you can get by without doing this, so I did not replace my O rings or oil. So far, so good. But I wonder how they put R134 in there in the first place?? Or WHY. R12 fittrings tell you to put R12 in or convert. I would seek additional information on this. Maybe the compressor was due to expire anyway, but, they are sensitive to junk in the lines, and I bet to mixed refrigerant. Check with some of the more popular forums, such as Blazer etc., even though you don't own one. GM is GM and the more popular forums will have a lot more experianced people in it. And, they don't mind answering questions. GOOD LUCK.