Going of ver recommended TBO

I've had a 172M for 31 years, now with 1700 hrs TTSN that still makes
compression (which never never has been that great) and still gets over
25 hrs/qt. It has been preheated religiously, run on Shell 15W50,
mostly autofuel, & mostly cruised at ~55% power. Oil analysis still
shows low and declining ppm metal contamination. It has a full flow
filter.

Given your operating conditions, you shouldn't break anything from
simple fatigue. You will eventually wear it out at some prolonged
point, corrode it internally or damage it on a cold start. All these
should give warning.

But I don't think you can make 2X TBO. You should critically look for
any signs of deterioration as you approach or exceed TBO. I assume
you have a full flow filter. Given the quality problems the
manufacturers seem to be having with recent OH components, I'd think it
safe vs an unnecessary overhaul.

Let me guess it is also an O-320 in a Cessna installation. I think
Cessna has better cooling provisions than Piper or especially Grumman
etc.