EPDM jacketed welding cable is oil resistant and has the same heat rating as SGT wire. SGX has a higher rating at 125C compared to 105C for the sgt and most welding. I have seen one EPDM jacketed cable that was rated at 140C

There is cable that is sae J1127 rated 105C that is welding cable with an EPDM jacket. My guess, it's the same stuff as the non SAE rated wire, just with the SAE number on it. Same specs throughout. Wouldn't surprise me if the jacket between the two had no molecular composition changes. The 140C cable must have something different for jacketing.

Yes on something like TechFlex jacket when it passes near headers. Heat hurt load carrying capability of wire. That goes for any of the wires headed to a starter that is essentially captured by headers.