R12 system charge with R134a filled TXV in MVAC... what would happen?

I asked auto AC people and its beyond their knowledge, so I bring it on here.

I'm working on a car that uses a VCTXV design and it was originally fitted with a R12 TXV, which was presumably filled with R12.

The old expansion valve was filled with debris so it was replaced, however the 4 Seasons replacement was stamped "R134a" and it is the only one available for this car.

Looking at this chart R134a needs about 2°F more to match R12 pressure in evaporator temperature region.


The system is charged with R12.
So... this means that when the evaporator exit is at 42 degrees, I think the expansion valve think its 40 degrees. because the TXV bellow gas R134a is 37 psi @ 42F and R12 is 37 psi @ 40F.

The super heat is only 4-6 degrees by design, I think. Would this cause the super heat to 2F higher than it should? Can I back off the spring set screw to try to match it?

To adjust this TXV, I have to access the adjustment hollow hex nut from the supply side of TXV with the line removed.

Why charge with an illegal gas, why not use the R134a the TXV is meant for?

It isn't illegal here. The system is R12.
R12 non barrier hose
R12 oil
single flow serpentine condenser.
It's just that replacement TXV is only 134a.

Just be careful that you do not get liquid back to the compressor. It is going to be quite time consuming having to reclaim, adjust the TXV and then vac out and re-charge. Don't forget that every time you reclaim, you remove a small quantity of oil as well. So this will need to be added back into the system. If this is your car and time is not being paid for, then its up to you.
If it is a customers car and you are charging them for many hours work, then you best forewarn them to expect a large bill.
On the other hand, for a pre -1995 car, maybe you will just have to accept that the system is not quite as effective as it was before.