I use a flare nut wrench to back up the oil gauge while tightening the
actual oil gauge line using a 10mm.....I learned my lesson the hard way (diesel oil soaked floor board, blue jeans and brand new boots...expensive lesson).

From memory (10 years ago), that line has a very small fitting and the torque required to seat the nut is very low. Do you really need a flare nut wrench on the gauge for it?

I use the flare nut wrench to hold the stationary bolt that is permanently attached to the back of the oil pressure gauge itself. This is so when I tighten the 10mm oil pressure gauge line (which goes into the oil gauge), I don't put too much force or tension on the action fitting. Last time I used only the 10mm wrench to simply tighten the oil gauge line, but must have used too much force which broke the weld and cause the oil gauge to leak a lot of oil.

then, I had to source an oil pressure gauge which required a lot of time and effort; I actually purchased a back up unit in case I make the same stupid mistake.

I suppose my question is whether the gauge actually needs so much torque to prevent any leakage from the fitting? These fittings have a tapered seat and seal with relatively low torque. The forces within the fitting itself are very high despite the low torque from the wrench.

OK, I understand.