I ended up using the 87Celsius version for now.
The thermostat that came out of the car was manufactured in April 2008 and it got installed on this d24t over 6years ago.
I don't think it started malfunctioning but I decided to have a new thermostat for the car.
Every previous Winters, when weather got colder, around thermostat housing the cast iron was usually a little "wet" and tightening the bolts didn't cure the leak (after removing the unit I could see why: as the o-ring aged, it got harder and "flat" ). No visible sign of discoloration/rust/corrosion and the inner rubber seal looks fine. I think Zerex G-05 is great quality coolant!
No loose particles or rust or anything was found within the cooling system so that makes me happy already.
I think (but i'm not completely sure) that the car now is able to produce better heat now (or at least earlier* than before).
For the Summer months I think I'll use the 80°C version.
Refilling coolant then bleeding all air out of the entire cooling system went really well this time (despite nightmare stories I read) so
I don't mind repeating this project soon. I would like to replace the rear heater hose anyway (the one that is right by the firewall behind the cylinder head) so probably I could add new Mackay upper and lower radiator hoses too. I believe one should not keep fixing things that are unbroken but I assume all our d24 rubber components occasionally get chemicals on them, chemicals that are obviously harmful: diesel fuel vapor from leaky injection pumps or return lines, mist of coolant or leaky power steering fluid (next fix
) , engine cleaning fluids such as soapy water etc and dont forget about stuff that gets under the hood, coming off the road surface when highway driving in rainy weather...also the salt and de-iceing liquid chemicals they use nowdays to melt the snow...straight onto your lower radiator hose. Filter changes also add some fuel onto your rubber
or when you install your injectors with new nozzles there us so much mess around!
)
I don't think 5-8years should kill rubber hoses but I feel better knowing if they got replaced/checked at least. Now, hopefully soon, fabricating a coolant reservoir with a level sensor....a long wanted item.
Back to the thermostat...
I read it somewhere on this forum that long ago these thermostats used to have an arrow (or a hole?) to show proper orientation so it would get installed in a specific/proper position (hole pointing up etc) although I haven't found any instructions on/in the box.