Just doing some reading on the links posted. It appears using a stainless filler rod is not a good idea at all.
"Making an analysis of the resulting weld chemistry for varying amounts of filler metal dilution creates a scary scenario at low amounts of any stainless filler alloy. When I discussed the use of stainless filler metal making these small fillets in 4130 tubing with a friend who is an acknowledged "worldwide stainless welding expert," he cringed! As he said, the suggestion that 312 stainless filler be used is based on at least 40 to 50% filler metal diluted in the high carbon material. If you make almost an autogenous TIG weld (no filler metal) and add just 20% of even 312 stainless you get a Martensitic deposit. You do not obtain the desired microstructure on which folks base their recommendation for a particular stainless alloy rod being acceptable. I have had race car fabricators say they like to use stainless filler because it makes the weld stand out and look good on unpainted frames they sell! Not a good reason since it could also contain cracks!"
Im not 100% sure what your expert is saying but it sounds to me like hes got it the wrong way round. 312 will remain austenite / ferrite up to around 50% dilution from the 4130 or mild steel. In other words it will be ductile at dilution levels in excess of what is possible with stick and TIG welds (using a filler).