I understand all the cademic aspect of thermostat, i.e., exactly how it works but my practical side tells me to replace the tstat every 90K-100K no matter what.

And yes, I only use Wahler brand, which is original mfg.

PS: You are lucky your tstat failed "open". If it failed "closed", you could have overheated the engine! Once your tstat fails, time to pay attention to the rest of cooling system. I wrote a "Partial Cooling Overhaul DIY" in forum, you can search for it.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT