These threads always end up being rife with misinformation and anecdotal hillbilly conversions.

Yes, some folks merely charge the system with R-134A with no other changes but this a simple case of doing it wrong and getting away with it.

There is no such thing as a "drop-in" replacement for R-12, a true drop-in would be a single component refrigerant and all of the so-called drop-ins are blends of other refrigerants or hydro carbon (explosive) mixtures.

The problem with blends is the size of the molecules, the different components in the blend have different sized molecules so, when the charge leaks off (all automotive systems leak), this makes the blend lopsided so you can't simply top off the system.

Again, R-12 systems deserve R-12.