It seems that most folks are going for R409A or more lately this HotShot stuff.
If you are forced to retrofit an existing R12 low temp system MP66 is the best interim replacement performer out there, in my opinion.
That being said, if you have an existing R12 walk-in freezer it must be nearly as old as dirt and long overdue for a complete system changeout. Sell the customer on a shiny new R404A system, collect your money and sleep well at night knowing you've done the right thing.
Actually this walk in freezer was installed in 1999. Tecumseh condensing unit but no where on this unit is their anything identifying what r12 alternative that they used. Its just listed as r12. Its in very new condition nothing very old. I have no idea why they installed an r12 system over 404.
Have not run across the need for MP66 in several years, glad for that. I'd recommend as previous and suggest complete replacement. They must've purchased some unit that sat for many years at the suppliers.
Watts New, Ohm My, I been Electrically Commutated. Are U2.