Grant Boiler Leakage Problems?

Can't find it now but yes, I read that the Vortex primary HEX is mild steel, the secondary, condensing HEX is stainless with aluminium (alloy?) turbulators which I presume may corrode but will hardly cause a water leak. Grant stated in the link I read that the return water temperature must not fall below 40C to prevent corrosion to the "boiler jacket" which I take it to mean the mild steel primary section. Don't know are they talking about jacket internal corrosion or one would think, external corrosion if the flue gases are below the dew point. Hard to see external corrosion, surely the flue gases leaving the primary are up around 200 or even 300C, somewhere around what one gets froim a non condensing boiler like my own Firebird @ 330/350C, even if the return water was as low as 25/30C, those flue gases shouldn't remotely be any where near the dew point.

@jimf did say that he has measured the primary HEX leaving gas temperatur by inserting the analyser probe in between the two exchangers, but I can't remember his readings?, also, just how much condensing is actually going on at all on these oil fired boilers as I think again, jimf said something I think in the range 85C to 110C for the boiler leaving flue gas temperatures.