Freeze 12 works wonders. I used freeze 12 in my old camaro, and it blew 60 degree a/c. Worked for me
I would not suggest mixing the refrigerants. You'll lock up your compressor that way. If you want to do the conversion, make sure your a/c lines are in decent shape. R134 molecules are smaller than those of r12. If the lines are in bad shape, you'll lose your 134 extremely fast. If you do get the conversion, the can of pag oil that comes with the kit, is not enough. buy another can, and run both of those through just to be sure.
By the way, I don't think 134 came in nissans until 95.