Even if you have r134A seals in the compressor and the oil is 134a compatible, you still don't have PTFE lined pipes so it's going to leak out faster than the drop in gases. According to the site below ST24 gives more capacity and lower leak rates from old pipes, as well as not needing any other components changed. I got 4 years out of a fill on the 610 (which is about the same as my newer cars)
My experience was that the RS24 performance dropped off more than the pressure, leaving pressure but poor thermal capacity, but still quite a pressurised system, a regas restored things impressively, and in these cars, its very welcome (if you live in the South).
When I lived in Scotland, my new car was nearly out of warranty before I found the AC didn't work...there were missing parts.
According to this site, the actual cost shouldn't be 100% more than 134a (more like £10 more) but it could be out of date, I paid around 120 a fill vs 35-50 for my 134a cars.
This probably doesn't allow for the HUGE gas capacity of our cars...900grams vs 380 in my Audi, which will increase the gap.
SO...I'd recommend the RS24, probably cheaper if you are keeping the car or if it doesn't have a new receiver drier.
http://www.kwecars.com/technical/air-co ... sion-myth/A bit more stolen from the MX5 forum...for all you chemists out there...
"R134 does have smaller and lighter molecules than the older R12.
R134a is 1,1,1,2-tetrafluoroethane, and has a molecular mass of 102g/mol.
R12 is dichlorodifluoromethane, which has a molecular mass of 120g/mol.
R12 is therefore very close to being a fifth heavier, and whilst R134a has an additional carbon atom compared to R12, R-12 has two massive great chlorine atoms attached to the central carbon which are both around twice as large as fluorine atoms and nearly three times larger than carbon atoms.
So yes, a 15 or 20 year old refrigeration system with tired seals will let R134a leave the system much more readily than it would let R12 escape, in the same way that if you take two identically made balloons, one filled with helium and one filled with nitrogen to equal pressure, you will see the helium balloon deflates much more rapidly (helium molecules being much smaller than nitrogen ones)."
More here (inc MSDS)
http://www.star-international.co.uk/ind ... ices/rs-24Lee