as said above, the proper way is 309L, BUT...
I've had good success with non critical things being held together by using regular mild steel ER70 filler, both MIG and TIG, for mild-to-stainless.
Using stainless 316L rod on mild steel does not work very well, though. Not in my experience. Creates a very brittle joint.
Ray, 309MoL is often the appropriate grade of consumable in this situation. If you search on that phrase you’ll find various manufacturers’ datasheets with recommendations on gas, parameters and the more subtle aspects affecting dilution into the base material. The old Avesta handbook (online in various places) has great guidance.
Michael
Michael, great information in that handbook, thanks for posting.
Sorry if this has been covered before. If you need to weld stainless steel to mild steel do you use stainless wire and appropriate gas? Also are the settings similar to what you would use for welding mild to mild. Thanks.