R-134a and R-1234yf are different products with different fittings and different oils, and they cannot be mixed.
R-1234yf systems operate at a different pressure and use a different lubricant. Using R-134a in a R-1234yf system, or the reverse, damages the compressor and creates a safety issue because the systems are not designed for the same pressures.
A system that needs topping up is a system with a leak. Refrigerant does not wear out; if the charge has dropped, the leak has to be found and repaired, otherwise the new charge escapes the same way.
Containers should be stored upright and away from heat. A container that has been overheated should not be used.
