Not fun in or under the engine bay! I think you may have to carefully drill it out starting with small bits and stepping it up. May have to super glue tack the bolt so it does not spin whilst drilling after a brake cleaner cleanup. When you have got close to the 6mm size you maybe able to re-tap it. Hope you don't need a helicoil.
Either it was a low tensile bolt or more than likely pvo just kept tightening it until the thread was stuffed, they only need 21ftpounds torque.
I just put on a restored oil pan on the 2h engine I am building. It is m6 1.25pitch. Having the correct taps for toyota is very handy to have, use them frequently.
Unlike some oem enthusiasts , sometimes I use something which I consider an upgrade. Where there is high chance rust damage for the bolts with low torque values I have been using 316 stainless bolts. Such as thermostat housing, oil cooler, oil pan. Admittedly they are 13mm heads but they are not usually pieces which need to be pulled on and off frequently. Some folks hate the 13mm. Stainless is soft and even easier to strip, but I really observe most damage done to engines and bodies is over tightening threads. I may get some flack for this!
In your circumstance I would keep it consistent with what you already have. Unsure how successful a seal would be obtained from replacing a single bolt on the pan. It may continue to leak until the whole gasket and goop is replaced as oil would have penetrated beyond the seal of one bolt.
This is why indie mechanics tend to leave leaks and just top up oil, unless going for a rwc. It is not best practice but sure is easier.
If you want to do it once for sure and already have a happy engine, take it all off and do it proper. Can clean up your lower end at the same time. Gasket and goop is cheap. Costs in time and love.
JIS sems bolts can be a pain to source, unless you pay for it through toyota or spend a few hours in the Japanese isle in a pickapart. MXstore and Ebay has them, but make sure they are at least tensile.
